Crazy but true...
Some years ago trainee teachers were monitored on teaching practice by their contemporaries, who sat at the back of the class taking notes (or not, as the case may be), and often snorting and giggling worse than the pupils.
One group of trainees set each other a forfeit: they had to introduce a wholly irrelevant word into their talk to the class: "wardrobe" in a biology lesson, perhaps. It had to be included in a proper sentence as part of an explanation. It always caught pupils off guard but worse was the response from the back: mirth that could neither be criticised nor acknowledged. The penalty for not getting the word into the lesson was the traditional one: the first round at the pub afterwards.
Once there was a newly qualified teacher who was locked in a stationery cupboard by his class.The teacher's classroom was linked to the neighbouring classroom by a big stationery cupboard, with doors opening into it from each room. The new member of staff was something of an anorak, and regarded as such by his class. They arranged to have someone in the next classroom lock the door when the teacher went into the cupboard for supplies. Then they locked the second door. He was stuck for the 20 or so minutes of registration.
The really silly part is that in the pub after school, Mr Nerd was full of it, though anyone with any sense would have kept quiet. He brought out the whole story for the other members of staff to comment upon. He was loud in his fury and eager to work out how to bring the perpetrators to justice.
His colleagues tried to persuade him that this was both impractical and counter-productive but he simply refused to understand. It took a couple of exhausting hours before he accepted that the incident would be better buried, and then only because the teachers wore him down.