Here's the most horrible/awesome/cartmanesque idea for a april fool's joke for a teacher. I find a kid who's been a dick to me, and I look back at the work he did earlier in the year, when we did a bunch of "getting to know you" activities, and I divine from these papers whether or not he has a pet. If he does, good, because then I can have another kid come in and hand me a note from the office (o.k. really I wrote) that says simply printed out:
Your dog is dead. We're very sorry, we love you son.
Mom and dad.
Then, when the kid starts crying, I start yelling at him and telling him he should take it like a man, and then finally, I tell him, "Ha ha, just kidding!" and we are all friends again.