We gave a teacher a nervous breakdown once. Age 13 we thought it was hilarious but as an adult I feel very guilty about our collective behaviour.
Not sure what they're called where you are; 'Rio Snappers' here. Little twists of paper with some sort of mild explosive in, you throw 'em at the floor and they go 'SNAP'. Everytime she'd turn to write on the board someone would chuck one, 'SNAP'. Reckon she must've been struggling already and we tipped her over the edge. Her husband was our history teacher and we paid dearly for our transgression in every lesson thereafter.
My best friend in high school had elbows that naturally bent backwards at a 45° angle. Any time we had a substitute in gym class, he would go running up to them holding his arm bent like that and screaming. The sub would freak out and go running off to the nurse with him. Cruel but also pretty funny.