Domain Users becoming Domain ADMINs

We have a Windows 2000 server with approx 700 users in our school. Periodically all users in our teacher group and student group are being added to the domain Admin group. There is no evidence in any of the logs. The time is never the same. It can occur 1-3 times a day or not for over a month. We think that all our security settings are ok but now we are stumped.