There were the times my brother and I slipped sheets of lead into schoolmates backpacks to see how long it would take them to wonder why they were so heavy... to to drop them abruptly when they tried to pick them up with ease and couldn't.
My entire class once hid from the spanish teacher in our classroom... she came in, and saw a few people - some of the hiding places were very obvious. I, however, was hiding in one of the cubbies (junior high lockers) that didn't face into the classroom, and I was small enough that I fit, and couldn't be seen. She kept asking where I was, and eventually went out of the room to ask about me... at which point I crept out and took my seat. I got a talking to later, though.
In the same school, I made the delightful discovery that our principal was afraid of mice - so I went to the pet supply shop and bought a couple of the very realistic looking toy mice. First, I walked up behind her with it in my hand, tapped her on the shoulder and held out my hand, to watch her shriek and jump backwards... then, for my grand finale, I hid it on her desk, next to her coffee cup, and waited outside her office as she talked to a colleague, and then caught sight of it... I got caught because I was laughing so hard at her scream, but she forgave me.
I did a few things in high school, but usually not in school itself - things like breaking into the old parking garage and flashing lights from the top of it at the passing police cars, or playing mind games with stoned students late at night. My teachers tended to be funnier themselves, like the time my physics teacher was showing us the effect that a strong magnet has on TV screens, and blacked out a chunk of the screen for a week.
My very favourite from that time was adding 'of pancakes' to the lettering on the door of Dartmouth's International House dormitory. I was pleased to find that, years later, when I was hanging out at one of the frats, when tales of practical jokes were being told, that one came up as 'no one knows who did this, but isn't it great?' I was proud to take the credit.