One time the music department bought keyboards for everyone. We immediately discovered that these keyboards could make synthesized cow and sheep noises. Naturally every class from that point onwards was soundtracked by 25 farmyard noises going off all at once, for 30 minutes. It was just relentless. I remember the teacher literally running out of the classroom crying.
Our school was chosen to host the annual teachers' conference day because we were the 'best behaved in the borough'. So the boys in my class let all the teachers' tires down and one pooed on the headmistress' windscreen.
Our RE teacher inexplicably decided to tell us that due to his religious beliefs, he had never had sexual intercourse, as he wasn't yet married.Everywhere he went for the next four years was accompanied by a chorus of pupils humming 'Like A Virgin'.
Found out two teachers were having an affair, kept whispering nonsense in the male one's lesson but occasionally raising our voices to say the name of the female one. The paranoia drove him mad, he threw a pen at someone, and it broke a window.
Our Spanish teacher was generally pretty stressy and needed all the chairs to be under the desks, straight before she would teach us. so obviously we would move them sliiiiightly whenever she went out of the room. One day we put them all up on the desks and turned the lights out and sat there and acted as if it was totally normal. I think she refused to teach us after that (but she did hug me when I was the only one in the class to get an A* when our GCSE results came out).Also we once locked our RE teacher in a cupboard.
In 5th grade (11-12 years old in the States) my friend and I got detention during recess. We were so mad at our teacher we put 409 cleaner in his coffee. Then we realized we could actually poison him so we washed out his coffee mug. Almost killed him though ;)
We had a German teacher who tried to keep us back after class at the end of the day. We had buses to catch, so we were pretty resistant to this. Our reaction was to basically throw our chairs at her and walk out. This poor old German lady was basically buried beneath about 15 plastic chairs. I feel bad about it now. But at least we got our buses home.
I was the slowest at cross country in my class. In year 8, everyone decided it would be hilarious if I won because it would mess up our athletics league tables. So everyone else ran purposely slow and fixed the result so I won. Even though the teacher was angry, she had to present the trophy to me in assembly, and my whole year had snuck in party poppers, which they popped and gave me a standing ovation. The teacher was the angriest of women.
We once drained out these creepy old preserved things we had in a science room and left a puppy (I think) fetus in a teachers bag. Reason for this was that she had caught us hair spraying our names on desks and setting fire to them, thus gave us a telling off.WTF to convent girls schools.