Put a Volcano 4000 in someones locker, when someone opens it.... BOOM.
Spitballs is also kinda a good idea, spit it out of a straw and then the victim blames it on someone else, which leads to a fight ^_^
When someone is running, you have the choice to put your foot out, causing them to fall over.
There are loads more, those are what comes off the top of my head.