get the locker of someone you really hate & fil it with bees. just get a ziplock bag and put sugar in it then put it somewhere where you know bees will go, when there are a fair few in there, close the bag (dont make it airtight, you want them alive) then go to the guys locker, open the bag a little bit & feed them through the hole, then put tape over the hole so they cant get out.
or: (this is good for a senior prank, not really april fools) get a bunch of people to dress up in like monkey suits or something, pull up to the gate in a nondescript van, looking all sketchy and just run into the school and generally cause havoc and stuff for like 5minutes. cos youre in disguise they wont know youre students so people will be chasing you and calling the cops & stuff. just be really organised & be like "in & out in 5 minutes"