Here's a couple that happened I know about some are just stupid though.
1) Take out the cream filling in Oreos and replace it with toothpaste. I'm not sure if you can die or get seriously sick from eating toothpaste so this one might not be a good idea but at least they'll have minty breath.
2) Take 4 pigs labeled 1,2, and 4 and set them loose in the school. Eventually the administration will catch all three but they will spend the rest of the day looking for number 3.
3)
>>9 is fucked up!
4) Take a beer can and put it on one of those shake'em up thingies like in the Simpsons and put it back in the fridge. Then knowing that your dad will open it for sure, be there when he does so so you could see it blow up in his face. When part did it though his dad went into a coma so be careful with that one.
5) If you're a dad then just tell your kids that you and your wife are getting a divorce. Then when they get all teary eyed yell out "April Fools" and laugh it up.
6) Bring what looks like a dog turd to school and eat it in front of the whole class. Then when done eating lick your fingers afterwards and yell out "April Fools!" but the only problem is what looks enough like a dog turd....maybe a melted snickers?