I'm in grade 11, but I know that the grad class is going to padlock the school so no one will be able to get in.
Before that, they layed bricks in front of the Principal's office, and cemented them.
Also, one class glued all the locks shut. It costs thousands of dollars to repair. Their prom was taken away.
And at a nearby school, they set their gym on fire.
I wouldn't recommend that either. Though yours has been done. Be creative!