Author's Note: Inspired by Largo's story, the idea of Ranma becoming Team 7's teacher because Kakashi is late just wouldn't leave me alone. This is the result.
Naruto did not like waiting. In fact, he hated waiting, as it took away time he could use for important things, like training to become Hokage and ramen. So, it did not truly surprise anyone in his new team that after waiting for their teacher for over an hour, he resorted to rigging up a simple trap, putting the chalkboard eraser in the space between the door and the wall.
It did surprise them, however, when he suddenly scrambled away from the door as if it tried to bite him.
Ever since Saffron and the fight at Jusendo, Ranma knew there were still opponents stronger than him out there, people, if he could really call something like Saffron a person, that he simply could not beat without resorting to killing. He had barely beaten Saffron at all. That knowledge did not sit well with him. So, ever since then, he had spent any free time he could scrounge up, which was depressingly little with all the rivals and fiancees orbiting around him and making a mess of his already crazy life, to try and learn techniques that he could use to tip the scales in the case of another overpowered opponent.
The first problem with this idea was that, aside from his father, which had already taught him everything he had (unless he was hiding something else like the sealed techniques), he couldn't think of anyone who knew techniques on his level and was willing to teach them to him. For that, his solution was obvious. His usual strategy is to copy and steal anything his enemies use against him, then improve on it and beat them with that will once more prove itself.
The first things to come to mind were Ryoga's Bakusai Tenketsu and Mousse's Hidden Weapons.
He already knew the training for the Bakusai Tenketsu and the theory behind it, which made learning that easy, if very painful. He had a few ideas for improving on it, but he would keep that for after the memory of the pain caused by the training disappears. Hidden Weapons was a lot harder.
The problem with learning Mousse's technique was that he simply had no idea what it actually was, only what it did. Every Martial Artist beyond a certain level used it on a minor, instinctual level, hell, even Akane had regularly employed it to store her Mallets (and Ranma knew for sure there was a lot more than a single Mallet in there). What Mousse did went far beyond that, and after some careful watching when Mousse had last attacked him, Ranma believed he was actually opening and closing minor gateways to his personal ki sort-of-pockets inside of his robes. This allowed him to form, for about a second, a portal that could theoretically contain any ranged attack, or another that would let out something that went in the first one.
The technique was far too valuable not to learn, and he was simply lucky that Mousse never thought to use it to do more than store weapons and spare glasses.
Of course, he couldn't get Mousse to teach it to him, so he had to try and figure it out on his own.
A full week of spending all of his precious free time putting things in and out of his ki pockets, and he was starting to get a feel for how to open a gateway. Of course, that meant he tried using Hidden Weapons to store anything and everything he owned, and when that really didn't bother him much he simply tried stuffing every boulder he saw in his ki-pockets.
That attempt brought an obvious flaw to the technique to his attention. He could not, no matter how much he tried, store any boulders bigger then Happosai is tall using the technique without breaking them apart first. Of course, knowing Akane's Mallets were often bigger than that, he went about trying to physically stretch the gateways. The first problem with that was that keeping the gateways open for any significant length of time was very, very draining. The second was that if he managed to stretch one gateway, it didn't change any of the gateways he opened later. On a much more positive note, Ranma had been very glad to discover that things could only be fully inside the ki-pockets or fully outside of them, not in-between, which meant he could safely use his hands to do the stretching without risking losing fingers.
Ranma was just about to give up and chalk that issue up to repetition and practice, when he noticed that the exit gateways could spew out many things at once, with no such size limitations. After that, he had simply stopped using the entry gateways to store more than air, instead using exit gateways to expel large masses of air, allowing him to fit larger things through while air was still coming out.
Of course, nothing ever goes smoothly for Ranma, and, having gained the ability to form gateways big enough, he made the mistake of trying to enter himself so he could see where it actually led. Thinking back on it, Ranma really couldn't tell how he ever imagined that would be a good idea, what he was thinking to actually go through with it, or if he was thinking at all.
Regardless, he had found himself standing in what seemed to be an endless and lifeless plain, covered with everything he had put inside of his ki-pocket. Only instead of a sky, he saw a large mass of flowing, soft blue light. When the light reacted to his surprise, he had realized with shock it was his Ki replacing the sky here.
His next revelation was that he was very, very lucky he had stored large amounts of air in here, otherwise this would have been a very dangerous, and very short visit. One way or the other.
Of course, he didn't really know how to get out. After wandering around in search of an exit for some time, he discovered the damn place looped every time he left the area where there were things he put here, and that moving things farther from each other only gave him more space to wander around in.
Eventually, he tried opening a gateway again. To his surprise, it worked. When he left through it, however, he had no idea where he was. Or why he was now a girl with no water in sight.
Naruto's reaction to the door quickly became clear, as it suddenly sort-of-rippled, and a short, busty, redheaded girl walked through. Sakura jumped in surprise, while Sasuke tried to reach for a kunai and jump away while still sitting on the desk, and ended up falling backwards.
He clearly did throw a kunai somewhere in that, as the redhead was now holding it, blinking in obvious surprise, mumbling something about not playing with the fabric of reality carelessly again. Then Naruto, being himself, burst in excitement and Shouted.
"That was awesome! If I learn that Jutsu, old man Hokage will have to retire and give me that hat when I use it to appear in front of his face! Hey, sensei, teach me that Jutsu you just did!"
"Sensei! You're late! We were waiting for you all day!" Sakura added, releasing her frustration from having to wait long after all other teams had left with their new teachers.
Sasuke was eying the redhead suspiciously. "This is our sensei?" She did catch that kunai disturbingly easily. And he could admit to himself he did want to know what she did to get here like she did.
"Sensei? What makes you think I will teach you anything?" Ranma was confused. This, unfortunately was nothing new. The source of that confusion however, was. If these kids knew Ranma's reputation as a Martial Artist, they should have known his, or at the time, her training would be too harsh for them. If they didn't, however, why did they ask her to teach them?
"We were waiting for you for hours! We are ready for any training you can throw at us!" Naruto answered before Sakura or Sasuke could respond.
At this, Ranma smiled a predatory smile that disturbed both Sakura and Sasuke. 'Perfect! Now I can show them exactly why everyone calls my training insane, and they won't be in any shape to keep bothering me even if they don't give up outright...'
"Okay, if you really think you are ready for my training, I will test you! If you pass the test, I will teach you, but if you don't, you have to promise to stop bothering me."
"But we already passed the Genin exam!" Sakura protested.
"My training, my rules." Ranma concluded, with a tinge of satisfaction.
"We'll pass your test easily, and then you'll have to acknowledge us!" Naruto exclaimed
'Well, what would get these brats off of my back without hurting them...' Ranma was wondering, and then she had it. "Simple. Keep us with me all the way. Go!" And with that, Ranma jumped outside from the window, heading to the forest while passing on, over, and around buildings seemingly at random.
The three Genin only spent a moment to collect themselves, and they were off after her, roof hopping wherever they could. It was hard, but they managed to keep sight of Ranma for a while.
Then they got to the forest.
Ranma was honestly surprised. Kids their age normally can't do half that well, Martial Artists or not. Sure, she was holding back, but she was older and she was the best. Time to get to the fun part. 'Okay, we're at the forest, now I need to find some bees...'
For the first time in their memory, Naruto and Sasuke agreed completely and without question about something. "Our sensei is completely insane!" Naruto was screaming to be heard over the buzzing of millions of angry bees, constantly spamming shadow clones to lead bees away from them, while Sasuke looked like he strongly agreed, but was too busy breathing fire at the bees to talk. Sakura was hysterically screaming and trying to keep Naruto and Sasuke between her and the ever-growing swarm of angry bees.
When they got into the forest, it looked like they were catching up. Then their sensei threw a beehive at their feet, covering them with honey and dizzy angry bees. They were fortunate that they didn't stop running, as the bees quickly recovered and gave chase.
Screaming at their redheaded sensei that she was cheating was quickly written of as a waste of breath, as every few minutes, she threw another beehive at them. Really, the only reason they hadn't lost her yet was that red hair stood out very clearly in the middle of the forest.
For her part, Ranma was impressed. The pink-haired girl was dealing with the bees as well as she expected, but the black-haired kid caught her by surprise when he started making some weird handsigns and breathing fireballs at the bees that got too close to him. And then the blond in the orange jumpsuit started sending out clones to distract the bees. And by the way they disturbed the forest, they seemed real. Real, blood-and-flesh clones! If there was any way Ranma could learn that around here, she was staying for a while! This place was awesome!
Hours later, three new Genin were staring up at their redheaded teacher with pure hellish hatred.
"You're crazy." Sasuke wheezed out from his position on the ground. All three of them were lying on the ground, covered with bee stings, scratches from branches and bushes, battered and beaten and completely exhausted from an entire day of running away from bees, wolves, more bees, a bear, and even more bees. They didn't know there were that many bees in the forest! The wolves and the bear gave up when they started tree hopping, but the bees were as determined to hurt them as before, only they could also attack from below. They could clearly understand exactly why everyone in their class were scared of Shino. If fighting him was anything like this, they stood no chance.
"Ha!" Naruto managed, after getting his breath back. "We passed your stupid test, now you have to teach us!" Despite his condition, interestingly better than both others although he was stung more, Naruto was glowing with pride that they had managed to follow their sensei, regardless of anything that happened on the way.
Sakura had, fifteen minutes into the chase, managed to relax, and had since been keeping an eye on the swarm of bees, warning the group to kawarimi away whenever they got surrounded. Now she was too exhausted to talk, and a panicked voice in her head kept mumbling about attempted murder and torture, yet she was still satisfied. She had helped, she did her part, although a bit late, and helped Sasuke-kun!
Ranma was just leaning over them, smiling widely. "You pass." She was happy, and excited. She was now a teacher, and her students looked like they have potential! Even the pink-haired girl, once she calmed down, had made a good showing of strategical thinking, and revealed a replacement technique that looked much better than the one Konatsu used! All the new techniques! Maybe this wasn't such a stupid mistake to get here after all.
"Now that we will be working together, we need to introduce ourselves. I'm Ranma Saotome, heir to the the Anything Goes school of Martial Arts. Your turn."
"I'm Uzumaki Naruto, future Hokage, and I love ramen! Remember it!" Naruto exclaimed with energy he really shouldn't have had at the time.
"Uchiha Sasuke, top rookie."
"Haruno Sakura, top scorer on the written test." Sakura finished with a blushing glance that would have been obvious, if not for her cheeks being reddened from exertion anyways, towards the Uchiha kid.
"Well, now that we all know each other, get some sleep and meet me here tomorrow at eight in the morning so the real training can start." Ranma concluded with a smirk.
Somehow, all three Genin were sure this was going to hurt.