That brings back memories, I did exactly the same too. The logins were shared with the Windows machines so after nabbing the password of our maths teacher, a few friends and I went through his account vandalising his PowerPoint slides to add subtle nonsense. Poor guy didn't know what on earth was going on during his lectures. Also found out the 'debug' login for the switches and used it to rather destructively mess with the network, much to the chagrin of the sysadmins. Good times!