As an admin you can set different levels of access for users in Mac OS X. For Teachers you may want to let them access some system prefs and restrict others.
You can uncheck the admin access for a user account under 'Accounts' in the System Preferences. Under 'Parental controls' you can set what a user can and can't access. Wish I could stop my lot from changing the desktop picture. I know there's a way of doing it in the terminal, but I'd rather not go into all that gobbledy-gook.