A good one I've heard about: You know that...doohickey on the sink? The thing that you can spray water out of if the faucet is running. Put a piece of tape on the switch, so when the first person turns on the sink...wham. They get spraaaaayed.
A prank last year at my school was when somebody put a piece of tape over the faucet of all the sinks in the Boy's bathroom. I was one of the victims when I turned on the sink and got sprayed. The other victim was the Principal, so that prank didn't go over too well. >.>