My first student helper had access to the local admin password to help out on the system. When in Y12 he gave all his mates access to the system from thier personal laptops fron the common room. Only a problem in that it bypassed myself and interfered with the allocation of IP addresses - near allocation limit at the time.

The ICT Co-Ord also gave him a domain password when I was off once, not realising that was the entire domain password and that the kid then had acces to all teacher areas including sims, sen etc.