Just to be clear, Edward and Alphonse haven't tried Human Transmutation when they get their letters. It will happen before their second year.
Ed was trying to find a book he hadn't read yet in their father's study when he heard the tapping. Alphonse was with Winry playing with Den when he got hit with a letter.
Ed opened the window, and in flew an owl. Mouth agape, it took a good ten seconds for him to realize that said owl had a letter in it's talons. He approached the owl carefully before he figured out that the thing was tame...somewhat.
"Edward Elric, Hohenhiem's study..."
Now Ed was suspicious. How the hell did these people know exactly where he was?
"Dear Mr. Elric, you have been accepted at Hogwarts School of Witchcraft and Wizardry..."
Alphonse opened the door, and called out to his brother. Much to Ed's shock, his brother had also received a letter.
"Brother, what do we do? These people know where we live!" said Al.
"Hang on. We're sending a reply back."
Ed took out a pen and began to write a reply.
"To whom it may concern,
Could someone please send a representative to our house and explain the letters?
While we are willing to accept TAME owls, the fact that you knew where we live is a bit disturbing. It would ease some of the issues we have over the fact that you could pinpoint where we were precisely at the time the letter arrived.
Sincerely, Edward Elric."
The owl in the study took the letter with patience. It had been trained to carry letters.
Two days later a woman came to the house. She reminded Ed and Al forcefully of Teacher. But she didn't seem nearly as bad.
"May we help you ma'am?" asked Alphonse.
"I am looking for Edward and Alphonse Elric."
"That would be us."
"Are your parents home?"
"Our mother died half a year ago ma'am. Our father left us a long time ago," said Ed calmly.
While disturbed at that news, she still asked to come in. Alphonse lead her into the living room while Ed did his best to make a simple tea. Finally he gave up on the hot water and resorted to alchemy instead.
Instant hot tea.
Once they settled in, the woman looked at them both.
"My name is Professor McGonagall. I teach Transfiguration at Hogwarts. What is it you wish to know?"
"First, I would like to thank you for coming all this way," started Ed.
She gave him a look that brooked no nonsense.
"How did you know where we were, and are you telling the truth about magic," said Ed, deciding to cut to the chase.
"Ah. Each letter addressed to a potential student is charmed so that it can find them practically anywhere. As for whether I'm telling the truth..."
She turned into her animal form, before turning back and then transforming the table into a dog and back. That set the boys off.
She heard a word in five. The words she heard the most were 'alchemy', 'laws', and 'equivalent exchange'.
"How did you do that?" they asked finally.
"Magic. That was an example of transfiguration, the subject I teach at the school."
It took them another hour to square things away with McGonagall. First they had to get a legal guardian in order to attend the school. So they chose Granny Pinako. Winry was a bit miffed that she couldn't go.
Their magical guardian on the other hand, couldn't be resolved as easily. Fortunately they wouldn't need one until later.
Ed and Al were like kids in a candy store while they went down the Alley. McGonagall took them firmly into the bank. Neither of them looked at the goblins oddly. In fact, Ed was downright cheerful when he saw them.
Only Alphonse knew why.
Once they had their account set up (Hogwarts automatically gives orphans a thousand galleon account to start with) they went shopping.
Ed and Al had to be pulled away from the second hand bookshop. Much to her amusement, both had found several advanced transfiguration books at a cheap price. Including a small book about Animagi.
It took less time than she would have thought leading the boys around the alley. They held some interest in the apothecary, little interest in the Quidditch supplies, and no interest in the herbology shops.
Finally they reached Ollivanders.
She watched with great interest how the boys reacted.
As usual, Ollivander appeared without warning from close by. Both boys instinctively went into a defensive stance the second they realized he was there.
They calmed down once they realized he wasn't going to attack.
Alphonse sat quietly next to McGonagall as Ed went through hundreds of wands. Finally his hand came upon a wand that sparked gold.
"Interesting combination that. Wood from a rare three that only blooms in the full moonlight with an extremely rare Solar chimera. Eleven inches, perfect for delicate transfiguration work."
Alphonse got up and went through half as many wands as his brother. Edward had discreetly kept count of how many failed wands he had gone through.
McGonagall paid for their wands and told them bluntly "You may each have one pet. Owls, cats and toads are the acceptable animals...though some people are allowed to bring rats or mice."
Seeing the look that crossed their faces she was silently relieved that they wouldn't buy rats or mice.
(And the reason was quiet funny really. For every ten rats that entered the castle as pets, four usually went missing before Christmas. Most of the students usually blamed Mrs. Norris. She didn't feel the need to correct that misconception.)
Alphonse had the strangest look on his face as he looked through the kittens. Ed had a silly grin on his as he watched his brother.
Then he looked around and spotted an interesting egg. It was almost pure gold with hints of red and blue.
He picked it up and looked at it closer.
"That egg appeared nine months ago. Two galleons for the egg, and ten sickles for the kittens."
"Wait, kittens?" said Ed suspiciously.
He turned around to find his brother with two little kittens. One was a golden color while the other looked like someone had smashed it's face in. It was an interesting ginger color.
It was then that Al did something Ed hated with a passion. He turned the biggest, most sorrowful eyes on his brother.
"Gah!" cried Ed.
He was no match for those eyes.
They walked out of the shop with an egg and two kittens. One of the kittens (the ginger haired one) rode on Alphonse's shoulder. The gold one rode in Ed's pocket.
"So what are you going to name them?"
Al looked at the gold one and said "Eddy."
Ed looked at his little brother in confusion.
"He reminded me of you," shrugged Al.
"And ginger here?" he nodded his head to the other cat.
Al thought about that for a moment before he said "Chibi."
The ginger haired cat growled at him in warning.
The cat gave him a look.
"It means 'flame'."
The kitten began to purr.
Since they were stuck in the Leaky Cauldron until the day the train left, the two people watched.
It was fairly boring, until August came around. That was the day they met the most interesting person around. And his name was Harry.
"Hullo. I'm Ed and this is my little brother Al."
The rather timid boy held out his hand.
"I'm Harry and this is Hagrid."
"Cool. You guys heading for your supplies? We're really bored waiting until the school term starts. Mind if we join you?"
Eddy popped it's head out of his pocket and meowed.
"Oh yeah. This is Eddy."
Ed and Al took Harry on a grand tour of the alley. Hagrid was more than happy to let them show Harry around. He was busy recovering from the ride on the Gringotts carts.
Since Harry definitely had deeper pockets than they did, Ed and Al showed him the best places to buy his school things. They had been there long enough to pick up a few things.
Harry walked out of that alley with an Auror class trunk with multiple compartments, enough books to fill half the library in the trunk, and an owl courtesy of Hagrid. (And once Ed and Al learned that Harry's birthday had been the day before, they promptly introduced him into the world of alchemy. He was hooked.)
Alphonse and Ed were bored.
Ed had promptly brought out the book on the history of alchemy that he had discovered in the second hand book shop. It was their only copy.
It was Al who spotted Harry and waved to the boy. Harry brightened up immediately and headed straight for them. The brothers helped him put his trunk up before they decided to catch up on what had happened.
Harry actually fit in perfectly with the two brothers. Ed was as protective of the boy as he was of Al, and Al felt a kindred spirit in Harry.
Halfway into the ride, a red haired boy popped in.
"Mind if I sit here? Everywhere else is full," he claimed.
Al cocked his head.
"That's odd. According the Hogwarts: a History, the train is charmed to add compartments to accommodate the students," said Alphonse.
Eventually he left, but not before he somehow managed to annoy Ed so much that he got a black eye.
It wasn't long before they boys were pigging out on wizard candy. Ed had developed a love for chocolate frogs. The second they figured out that the frogs weren't actually alive, the other boys began to attack the pile as well. Ed kept gagging on the jelly beans.
"They mean every flavor," he said, choking. The others laughed at him as they experimented with the beans.
A girl popped in asking about a toad.
"I think there's an advanced spell that can summon things. Perhaps the upperclassmen know it?" suggested Ed.
"Of course! The summoning charm! I'm sure the Prefects know it!" she said excitedly.
She left after closing the door quietly.
Ed was busy playing poker with Harry and Al when the door opened for a third time.
The boy in the door was pale blond, arrogant, and annoyed Ed simply by being there. The goons on either side of the boy didn't look too bright at all.
"Argh! What do you want? I'm winning here!" said Ed in exasperation.
The pale blond lifted an eyebrow at the game. In the center of the makeshift table was a small pile of every flavored beans. Harry had the biggest pile, with Ed right behind him.
"They've been saying on the train that Harry Potter is arriving to Hogwarts."
Ed scowled, "What's your point? Who cares what his name is?"
"Clearly you don't know who you are talking to, mudblood. Everyone in our world knows who he is."
"Who is he then?" asked Ed with narrowed eyes.
"He's the Boy-who-lived, the only known survivor of the killing curse and the one who took out Voldemort."
"And?" said Ed.
The pale blond gave him an odd look.
"And he's the most famous underage wizard alive right now."
"I'm sorry, but what exactly is your point?" said Alphonse.
Apparently the boy had no reply to that. He had never met people who not only didn't know who Harry Potter was, but honestly could care less.
"Who are you anyway?" demanded Ed.
"Malfoy. Draco Malfoy," said the blond pompously.
"Edward Elric. This is Harry and my younger brother Alphonse."
"What of it?" said Ed in a growl.
"You aren't related to Hohenhiem of Light are you?" said Draco in shock.
"Hohenhiem was my dad's name, why?"
"You don't know? Hohenhiem of Light is renowned among potion-makers for discovering most of the uses for Chimera blood! And he discovered most of the valuable animals for potions!"
Clearly the boys didn't know much about their own father.
"I can't believe you didn't know."
"Our dad left us when we were little. We haven't seen him since, not even for our mother's funeral," said Ed darkly.
Luckily for the boys, Draco left quickly left after that. Of course he left with more questions than answers.
Namely why his godfather never mentioned that Hohenhiem's sons were on board.
Ed and Al were bored. Harry was nervous at first until Ed and Al started cracking jokes.
On the boat ride to the castle, Ed and Al made a new friend in a boy named Neville. Neville had a love for plants that he couldn't express because of his fear. He was terrified of his own grandmother.
Ed had chuckled at that and told Neville of their surrogate granny...who was known to throw wrenches at them when they messed up the laundry.
That had shocked the other person on the boat, the girl who had entered the cabin earlier. Her name was Hermione.
"Granny Pinako may be an old hag, but she's alright. She took us in when our mom died."
They entered the hall, and most of the eleven year old children were scared out of their wits or excited. Ed and Al were just bored. They half paid attention to what McGonagall said, before they prepared to help Harry.
The green eyed boy had caught their attention, and as far as they were concerned he was a part of the family.
One by one they sat on the stool to be sorted.
Hmm...loyal, hardworking and brave. There is only one place that would suit a gentle child like you.
Brave, rash, loyal and protective. The only real ambition that you have is to protect those who are family. You are best suited for...
Things went smoothly after that until...
Brave, loyal, cunning, and intelligent. You would fit into any house here! Which house would you like to go in? I am having a rather difficult time placing you.
Harry's mind brought forth the image of Alphonse and Ed. The golden haired boy acted like the older brother he had always wanted but never had.
Oh ho! So you have already formed a bond with these two? In that case, which one do you with to share a dorm with?
The roar was deafening as Harry timidly sat next to Ed. A pair of twins were doing the happy dance.
Ed and Harry ended up right next to each other. Harry had a silly little grin when he realized this.
They were asleep in minutes.
Ed woke up with the sun out of habit. Ever since the brothers had returned from Teacher's hellish training a few weeks after learning they had magic, the habit hadn't really wore off.
Harry was yawning and crawling out of bed. Ed raised an eyebrow.
"My family had me cooked all the meals, including breakfast."
That surprised Ed.
Both were in and out of the shower within thirty minutes. Harry had to help Ed with the tie, until he finally threw it down in annoyance.
"I am never wearing this!" Ed proclaimed, which had Harry laugh.
Fifteen minutes later, and Harry followed suit. He didn't really like the tie anyway.
Ed found his little brother at the Puff table, discussing gardening tips with Neville. The timid boy was thrilled to have someone his own age that loved plants as much as he did. Ed promptly crashed on the bench next to his brother.
"Ed! And...Harry? Why are you up so early?" asked Al.
"I slept in. normally I'm up by five," said Harry with a shrug. (The unofficial master of cooking while nearly half asleep and still getting the food perfect.)
Ed was literally growling as his stomach announced it wanted breakfast, and it wanted it now. Whoever cooked the food took the hint and it wasn't long before the boys were eating breakfast.
McGonagall frowned when she saw where her lions were, but still gave them their schedules.
Ed had a better sense of direction than Harry, and managed to get them to the class well before the bell began.
Though he was carrying at least three bags of food for a snack later. When he spotted the cat on the desk he grinned.
Ed put his things down and searched for something in his bag. His grin widened when he found it. In his hands was a cat toy that he had bought to play with the two kittens that Al had. (Though unofficially the gold one belonged to Ed.)
He walked up to the stern looking cat and moved the fluffy toy around it. It didn't take long for the cat to become interested and start attacking it. Harry laughed and joined in on the fun by finding some string.
The bell rang half an hour later, and they finally sat down. Once everyone was seated, the cat jumped from the desk and gave them all a shock!
The stern looking cat on the desk was McGonagall herself!
"Ten points to Gryffindor for kindness towards animals."
Ed's mouth shut loudly once he realized he had been playing with his teacher for the past thirty minutes. Harry was snickering at his expression.
Fifteen minutes into the lesson, Ron Weasly showed up, claiming to have gotten lost. Ed rolled his eyes at the lame excuse.
Charms was a rather interesting lesson, especially when the teacher fell off during roll call when he said Harry's name. Ed snickered then, and had his head in the book for half the lesson.
Potions. Most of the other Gryffindor students were dreading the lesson, as the older students had mentioned a teacher biased towards Slytherin. Which didn't help, because they were always paired with Slytherin in class.
Ed on the other hand, couldn't wait to go to class. Harry was skeptical, but once he learned what potions entailed, he was all for it.
He loved cooking.
Ed shivered. The potions classroom was cold, dank and had an odd smell. It didn't help that most of the other students had been whispering dark things about the teacher.
Though he did look at the odd jars around the room. He wondered how they could be used to create potions.
The door slammed open, and all talking stopped. Ed hurried back to his seat, hoping the teacher didn't notice him out of it.
Professor Snape gave his usual speech, ignoring the blond who snickered when he called most of his students dunderheads.
Then it all went to hell in a hand basket. First he started drilling questions at the Potter-spawn, then he trips over a large indent in the floor that definitely had not been there five seconds ago.
While he was getting up, he noted the odd striations on the stone. He recognized them after a moments thought.
He stifled a growl and continued to harass Potter, who had taken the time to answer his previous question correctly. Before Snape could turn, he felt something beneath his feet.
The brat wasn't going to pull it off twice! His foot came down, and he fell flat on his face. He looked where his foot was.
Whoever the alchemy brat was, he was good. Instead of doing the same trick from before, he had turned the stone beneath Snape's foot into sticky adhesive. Which impressed Snape somewhat.
That still wouldn't spare the brat a nasty detention when Snape finally caught him.
The lesson went on as planned, though Snape was in a foul mood when Weasly's potion exploded.
When Potions ended, three things were certain.
One, Snape hated alchemists who clearly didn't respect a teacher's authority.
Two, someone or something had managed to prank him twice in full view of impressionable eleven year old kids.
And three, if Snape found the culprit before McGonagall or Dumbledore, they were screwed sideways.
All in all, Ed considered it a perfect lesson, as he snickered the second he was well out of hearing range of the very irate Potions teacher.