Well here's a few ideas:
-The senior class ahead of us brought water guns into school and had a massive water gun war in the hallways
-The senior class before that came into school one day when it was closed and had a massive air-soft gun war in the school and left a mess for everyone the next day (our class didn't do anything, we were lame

)
-My cousin and his friends had the grand idea of egging their school. Just remember if you do that, don't be an idiot like my cousin and buy all the eggs 10 minutes before at the store down the street from your school so that they have proof that you did it. Be smart like me, and buy the eggs hours before at a store a few towns away xd
Some other ideas I would never do, but I heard about people who actually did it:
-Some seniors actually came into school when it was closed and made a brick wall in the door frame of the principals office, they got into a shitload of trouble
-And the most disgusting prank ever that I heard about was about how a bunch of senior guys got together, and well.....they all jerked off into a bucket and dumped it all over the principals desk. I don't know what happened to them, but it probably wasn't good, so I wouldn't do that if I was you U_U