My wife is an Administrator for a high school.
I asked her and she said these were three of her best ones in over 30 years in education :
Go to a bait shop and buy 200 crickets and release them in the commons area. Crickets won't multiply like roaches. She said they all appreciated this prank and no one got in trouble.
A student's father owned a construction company. They took the fathers lift/crane and put discarded or used tires all the way up the flagpole to the top with it. No one got in trouble on this one.
Another student put an ad in the paper as AN ESTATE FOR SALE BY OWNER. He described the school as having a indoor swimming pool, outdoor tennis courts, track, how many bathrooms, bedrooms (classrooms), weight rooms, basketball court, large yard, state of the art kitchen, "close to schools", personal office, etc etc.......let your imagination fly!!! Put the main phone number in the paper with the principal's extension #. A reasonably low price, 1,mil.......generated tons of calls to the principal. She liked that one the best, she said. Awarded for most creative.
ok, that's my wife's ideas. Have fun.