Man I miss high school so much...
I did not do it, but my one super techy friend in high school somehow accessed the admin features which accessed all of the computers in the school at once and put a very obscene picture of gay male sex on the desktop of every computer in the school. He was eventually caught and not allowed to participate in the graduation ceremony, but it was sooo funny.
This wasn't at graduation, but one normal day we also had people pulling the fire alarm and setting off the sprinklers, it was pretty bad since the teachers tried to continue class with the loud noises, eventually school was let out for the day, but it was funny.
XI Wiki

