GB, I guess you wanna hear it.
We got a very old and very bad woman as teacher for mathematics and accountant in business school. She had a ritual to go to the teachers toilette right after the hour in our class. One day I head the idea to place foil on the toilette. My best friend has taken the spare key for this toilette out of the teachers room before and we prepared both toilette's with the foil (because we didn´t know which one she will use). At least we taken out the light bulb so she can not recognize the foil. The rooms also just had a very small window so it was quite dark in there. What can I say, she gone the toilette after the hour and we all run after her and was waiting in front of the door. A loud screaming and she didn´t came out for approx half an hour

You have to know, if you pull up the toilette seat and put the foil directly on the ceramics, it works like a membran that pushes "water" back to you
For the last joke with this teacher the entire class was kicked out off school for a week (we stayed together because no one liked her!

)
To make it short - we placed a larger fire cracker right under her chair with tape (right in the hour when she sit on it!! As usual this woman recognized NOTHING

). We used the fuses from many other crackers and put them together. Fire on fuse and after a short while she was shouting "who burns stuff here? STOP IT". Some seconds later... BOOOM.
You have to know that the teachers chairs was made of wood with a very thin wood seat on it. The cracker did n´t harmed the wood but the shock wave is strong enough for this 3 mm wood plate to hurt you on places where the sun never shines

We had a lot of fun in this 7 days without school and in fact it was n´t a penalty for us

The teacher was send to retirement some time later because "she was too old to teach anymore" LOL