"Ah, Wizard Watkins, I wanted to see you. Did you teach 8B the invisibility spell yesterday?"
"Yes headmaster, we had great fun. They were slow to learn at first, but by the end of the lesson there wasn't a child to be seen."
"That's what I wanted to see you about. The invisible children have been wreaking havoc around the school, stealing spoons from the canteen, breaking in to teachers rooms, listening in to private conversations and possibly missing lessons."
"They're there for registration, but after that it's been impossible to tell."
"Well invisibility is on the curriculum, it's on of the Core Level 6 Wizard skills."
"Yes Watkins, but there's more to teaching young wizards than simply arming them with a cache of dangerous spells. You have to be careful what you teach children, otherwise you find yourself alone in a room with 30 of the blighters who you've just taught to make a fireball, turn their farts into poison gas or turn their teacher into a hamster."
"But those are all on the curriculum headmnaster. They're expecting to become wizards, it's what we're here to teach."
"There are other ways, Watkins, safer ways. If you concentrate more on the theory perhaps.
"But the exams, headmaster. The students have to pass a practical session. You can't give them their invisibility certificates if they're sitting there full-bodied."
"Honesly Watkins, that's easily fixed. You're invigilating the exams, just waft your wand at them at the key moment and turn them invisible. That's what I've always done, why do you think we have the best Wizarding Exam results in the country."
"Right, I think that's a clear enough confession."
"What do you mean, Watkins?"
"I'm not Watkins. I'm Walpole, from the Exams Office. I magicked myself to appear as Watkins to catch you out. You're under arrest headmaster, for falsifying magic."
"You're making a big mistake, Walpole. I'm not the headmaster. I'm one of the senior students. We've had the real headmaster locked in a cupboard for over a year now, we take turns to be headmaster for a day. It was just a little prank."
"So which pupil are you?"