My favorite April Fool's joke to play at school came from the Second Grade Teachers Club. In the morning I announce that later in the day we'll be having a flood drill. (The kids don't even ask questions at this point). When it is time for the "drill" I tell the kids that we need to treat it seriously like all other drills. They may not talk. When I give the signal, they have to take off their socks and shoes, and climb onto their chair. Then I announce April Fools. It's a hoot!
And of course there is always the Brownie joke. Each kid will receive their own brown "e" cut from paper. Nothing like humor that second graders appreciate.