lockpicking.
20$ gets you an electric lockpick that goes through any masterlock in 10 seconds.
as for the actual prank, dunno. for starters id do the powdered mashed taters + powdered milk on front lawn. sprinklers=instant taters.
oooh..... i hath figured it out. fog machine in ventilation system. wire it up and make a custom switch, sometime during the day go and flip the switch, viola. your school will be filled with smoke for about 10-20 minutes unless you create a custom resivoir for more liquid.
best thing is the smoke is totally non harmful to anything. now why is this good? if it was real smoke you pay for any damages done to the school. this way you freak everyone the fuck out, and dont have to pay for anythin if you get busted.