A/N Sorry for the delay. Here's the final part:

The sight of Fred and George's angry faces set Ginny laughing again and this time Professor McGonagall and Lockhart broke apart from their embrace and started laughing too.
"This isn't funny, Ginny!" Fred said crossly. "And you two can cut it out as well!"
"That's no way to speak to a teacher!" Professor McGonagall snapped, but the effect was spoiled by the twitching at the corner of her mouth as she desperately tried to keep a straight face. Lockhart - if it was Lockhart - nudged her sharply in the ribs. This, however, was a mistake. McGonagall spun round and jabbed him hard in the stomach, eliciting a yell of pain and causing him to double over in agony. As she stepped back, a smug expression on her face, she tripped over the hem of her robes which were suddenly inches too long for her. Tumbling forward, she grabbed hold of Lockhart's robe to steady herself but all she managed to do was pull him over as well, so that the two of them found themselves in an undignified heap on the floor.
"That was your fault!" Lockhart muttered irritably, his voice strangely high pitched.
"You started it!" McGonagall's hair was changing: instead of straight and jet black it was fading to a lighter brown, and the ends were starting to curl and stick out. Lockhart's hair on the other hand was getting darker and he, too, seemed to be shrinking: the robes that had previously been a perfect fit now hung well over his hands and feet. His eyes were slowly turning from blue to green and a strange mark was appearing on his forehead until suddenly he was bearing a startling resemblance to...
Harry turned a deep shade of red as he tried to disentangle himself from Lockhart's too-big robes. Before he could escape back upstairs Fred and George descended on him, grabbing the collar of his robes and pulling him roughly to his feet.
"I suppose you think this is funny!"
"Don't tell me - polyjuice potion."
"You could have got in serious trouble for this, Harry!"
"What if McGonagall - the real McGonagall - had come in?"
"Where d'you get the potion from, anyway?"
"Hermione made it," Harry mumbled. "Would you please let go of me?"
Fred released Harry but continued to glare angrily at him. By now Hermione had also got to her feet. She looked suitably ashamed of herself, though there was still a mischievous twinkle in her eye.
"So, Professor McGonagall... where's Professor Snape?" George's words were heavy with sarcasm.
"...or should we say Professor Weasley?" added Fred. "Where is our darling brother?"
Hermione blushed. "His potion wore off too quickly," she explained, as she gestured over to a cupboard on the far side of the room that was used during term time for the storage of broomsticks, footballs and other sporting equipment. Normally it was so full that the door barely shut; now at the end of the term it was empty. It would be an ideal hiding place for anyone who wanted to spy on the common room.
George strode angrily over to the cupboard and wrenched the door open. Sure enough, Ron was inside, a handkerchief stuffed in his mouth to muffle his laughter. He blinked his eyes furiously as daylight suddenly streamed into his previously darkened hiding place.
"Get out here!" yelled George, grabbing his younger brother's arm and hauling him out of the cupboard. "You're going to be in big trouble when we tell Mum about this."
"You can't tell Mum," Ron said defiantly. "Otherwise I'll tell her about your porno mag!"
"You wouldn't dare," Fred said, but his tone lacked confidence. "Whose idea was this little charade anyway?"
"Harry's" said Ron, at exactly the same moment as Harry said "Ron's." There was a momentary pause and then the whole story spilled out: how Hermione had come up with the idea of using polyjuice potion to quiz Malfoy about the Chamber of Secrets; how they'd distracted Snape so she could steal the ingredients she needed; how the potion had been brewed in Moaning Myrtle's bathroom and how their interrogation of Malfoy had failed to give them any useful information, save that it was not Malfoy who was responsible. Harry and Ron didn't mention Hermione's misadventure with the potion: one glance at her stony expression had been enough to warn them off. After that they'd more or less forgotten the potion until Hermione had reminded them about it that morning. There had been just enough left for the three of them and they'd decided to play a prank of their own on Fred and George. Hermione, after much persuasion, had eventually agreed. Ginny had not needed persuading: she had been delighted to take part.
Fred and George tried to remain disapproving but couldn't hide their awe at the trio's ingenuity.
"But where did you get the robes?" Fred wondered aloud.
"Easy," Ron said. "Stole them from the laundry. Hermione pretended she'd left her watch in her pocket so we had an excuse to sneak in."
"Lockhart must have forgotten to take his with him," Harry added.
"And there was hair on the collars of the robes," chimed in Hermione.
"Luckily it was the right hair!" Ron winked slyly at Harry and then yelped in pain as Hermione elbowed him.
"It still wasn't funny!" George said loudly, but then he remembered McGonagall - well, Hermione - declaring her undying love for Harry disguised as Lockhart. He couldn't help but laugh.
"Just you wait," Fred declared. "We'll think of a way to get back at you. And in the meantime these photos are going on the fire before they cause any more trouble."
He tossed the photo of Snape and Hermione onto the flames but then hesitated.
"Actually, I think we'll keep this one of McGonagall. Just for... er... potential blackmailing purposes."
"Whatever you say, Fred." Ron grinned wickedly. "Now get a move on or we're all going to miss the train!"