if you guys have locks on your lockers, you can run by with a screw driver and pop all of the dials off, so they will have to come and cut off all the locks, and no one will have locks.
you can also throw some smoke bombs, but for a prank like that dont tell anyone it was you, except for maybe your closest friend that wont tell anyone.
zip ties are fun, you can zip tie peoples backpacks closed, zip tie lockers
a funny prank we used to do is you grab someones backpack, take the stuff out, turn it inside out, then put the stuff back in and zip it up. Then you take a zip tie and lock the zippers together with it. Then if you wanna go one of 2 steps further, you can duct tape the shit out of the zippers so they have a hard time undoing it, and then duct tape the backpack into a giant cocoon.
its already been said up top, but spray pam on random places on the floor, or on door knobs.
i thought of the alarm clock deal once also, but my idea was to put them in your lockers, and set them all off at the same time, like 10 minutes after class starts, so the whole school will be disrupted
you could also just throw a bunch of bouncy balls or marbles all over the floor, just get 1000 marbles and let them fly
i could probably make up some more later