They try it on Hermione.

"Hey," said Nymphadora Tonks. "Can I have some of that?"

"Some of what?"

"That drink you're carrying."

"Oh, this can of soda? You can have the whole thing. I don't really like sweet drinks."

"Then why do you have it?"

"I found it in the common room. And it's hot today, so I thought I'd have it anyway, but if you want it it's all yours."

"Thanks!"

Tonks watched Hermione trot away down the corridor, her skirt swinging as she raced off to her next class. She looked like she was in a hurry to get somewhere. Which reminded Tonks that she was two minutes late to the guest lecture she was giving. She turned and ran the other way, swigging the entire can of soda as she went.

She burst in through the door of the lecture theatre, scattering notes everywhere, and jogged hastily up to the podium. The theatre was packed with students and teachers from all across the school, as well as several dozen of her colleagues from the Ministry of Magic, and even a bored-looking journalist who'd come to write it up for one of the less interesting wizard newspapers. His camera clicked as Tonks cleared her throat, ran her fingers through her pink hair and shuffled her papers around on her desk. She was nervous. This was her first time giving a lecture at Hogwarts, and she'd be incredibly embarrassed if anything went wrong.

"Hi," she said. "My name is Nymphadora Tonks, and I'm here today to talk about magical practical jokes and how to avoid them. Also, my name is Hermione Granger, and I'm a stupid little nerd who's going to take all her clothes off."

The journalist perked up.

Tonks unfastened her jeans. She turned around, pulled them down, bent over and wiggled her panty-clad bottom in the faces of the astonished audience. She took hold of the back of her panties and pulled them up tightly, giving herself a wedgie, before lowering them to her knees and smacking her bare bottom five or six times. "I'm a bad girl who needs a spanking!" she yelled, face suddenly very red. "Spank me, sir! I'm a naughty slut!"

Professor McGonagall rose from her seat, but Dumbledore laid a restraining hand on her knee. "This is obviously part of the lecture," he said. "Very interesting. Let's see where she takes it."

"Albus, you're a pervert."

"Not at all, Minerva. Have you forgotten I'm gay?"

"I do apologise, Albus. You're quite right, of course."

Tonks kicked off her jeans and panties, stood up and pulled her shirt off over her head. She spun around to face the crowd, legs parted, wearing nothing but her bra and boots. Her pussy hair was also pink, and cut very short. "Look at my pussy! I'm Hermione Granger!"

"Clearly," said McGonagall, "that is untrue."

"I know. What a fascinating presentation."

Tonks unsnapped the bra and twirled it around her head, doing a stupid little dance, before letting it fly into the crowd. Her smallish breasts hung free, and her face glowed incandescently red as she stared out into a sea of faces, all of them struggling to fight back grins. This was not going according to plan. When she figured who was doing this to her, she was going to make them pay. So much. They'd remember her vengeance for years. All she could do was hope that, now she was nude, they'd lose interest and let her...

"I'm a horny little harlot who loves to play with her pussy! Everyone look at me! I'm going to touch myself until I cum!"

"I don't understand," said Romilda, sitting in the next room, watching very intently as Hermione sat quietly in the row in front of her, listening to an incredibly boring history lecture. "I'm sending her mental commands right now. She should be up the front of the class, nude, humiliating herself in front of everybody."

"It worked so well on Ginny," said Lavender.

"Merlin's bollocks. Fred and George let us down again."