Back in 6th and 7th grade me and a few friends took a computer app class.
Bye some point in the year I had figured out how to delete almost all the files on the iMacs we were using, and I mean that I somehow removed the delete-protection for every file on the hard drive. I think I wiped out at least 5 brand new iMacs this way.
Nearly as much fun was doing something that would cause an admin to monitor our accounts, then start writing disturbing and frightening messages in text files. Oooh, and we would make gory flash movies and play them in loop on every screen while the rest of the class was still in lunch. When the class would come back, we would be coming in with them to watch their reactions.