You Americans have no clue about pranks haha ;-)
Just off the top of my head why not pour turbo lax into the drinking water dispenser in the teachers / principles offices, that way they will all be shitting through the eye of a needle!!
On the plus side this sets them up for stage 2.
Rim their toilet seat with super glue so when they go and do their anal fountain impression, they will also be glued to the seats.
(super glue or crazy glue as you call it) stays "wet" for ages until sandwiched between two surfaces ie the seat, and their ass so it's the perfect ambush weapon!!
Also while you have the glue....put some in the locks of the lockers to render them useless.
Another one is salt.....if you have a football field, pour "rock" salt on it, it had to be rocksalt though, draw what you like, "princie fanny flap is a bell end" or whatever, over the next few weeks it will kill the grass and become more and more obvious till the grass turns brown, in the meantime you seniors are well out of the picture while it works it's magic!!
If your not brave enough to diss the principle then just incriminate yourselves and put "class of 2010" or something.
Seafood hidden someplace maybe for example thrown into a ventilation / aircon system would be a pretty sweet idea fresh seafood smells neutral, but after a few days it starts to smell.
3-4 days and it goes to a whole new level!!
I could go on but i have to get some sleep to plan a prank on my wife.