At our school, the "computer" teachers were often teachers who were specialists in other areas that had some interest in computers, and weren't very ... security aware.
They all had admin/root user access, and they'd often forget to sign out, leaving us with the keys to the kingdom, at least temporarily.
We figured out how to create a SUID shell, so we could get back to root even after we head logged out. Poking a few bytes would have been more interesting!