My personal best April Fools joke:
Symphonic band tour in Florida, spring of '87 (back when you all were in elementary school

:). I got half the band in on it -- and all I did was tell one girl that I was really going to get her on April Fools day. Every time I talked to someone, we looked at her and laughed. She went absolutely nuts.
At midnight on a bus headed to (I think)Miami, she looked at me and said "You never did get me!" and I said "I got you all day long. You went nuts trying to figure out what I was going to do, and I did absolutely nothing. You spent the whole day paranoid for no reason."
Yeah, they did it on M*A*S*H first. But it was still funny. And the girl married me anyway, so it turned out fine.