personally i think you should do something that fucks with the administration and nothing the janitors will have to pick up. I say this because usually the administration is made up of a bunch of assholes and the janitors are usually just chill people who hate their fucking job. So don't make their day worse by making them do a shit ton of extra work that they hate.
But if you still feel like being an asshole I you should put copious amounts of rubber bands on the floors, like seriously tens of thousand. You have to pick them up by hand because they just roll up of you try to use a broom or shovel to pick them up. (I think I already posted this in another prank thread but whatever).