countless knuckle sandwiches/noogies
tying shoe laces together
many wedgies
many wet willys
cacking/debagging
many knocking and running
I can't remember anymore mild ones!!
and ohh favourites: one of our teachers was an absolute tyrant and once we managed to get into his room and broke his computer that was great
I'd always end up turning desks upside down when the teacher was out of the room. Geography class was mayhem, because me and the friend I sat next to hated the lesson, so we'd literally do something stupid every single lesson.
also when I emptied my friends schoolbag when he left for a while, and filled it up with random junk like wads of paper and colouring pens. BUT I put all his books and stuff in my bag so he didn't lose them because I am so kind :):))::)
then me and one of my friends used to annoy our maths teacher all the time; we overturned her desk by accident once, and made makeshift buzzers and kept making noises when she wasn't looking. Eventually his buzzer got stuck and he was smashing it on the table to try and make it stop buzzing, and didn't realise everyone was staring until after he'd thrown it out the window.
Best part was, he would always get caught and hated, and the rare time I got caught I was allowed to get away with it because I was a favourite student olol.
Best prank done to me was when somebody put a lunchbox full of mouldy food in my bag, and I opened my bag in class and was like "uuuuuuuuh whose lunchbox is this" and opened it and the entire class got stank out by what must have food rotting for months
I don't do it anymore though that stuff stays behind at school