Hi Dezz,
For all your settings, make sure that all the Macs have a seperate account for your students, not an admin account, just a user. Then only the admin can make changes in any system preference.
For your desktop in particular or any preference, one by one, you can click on the little lock on the bottom left hand side of the window to lock that preference from any changes.
Globalwise it would be easiest to make a seperate user account for all your Macs.
Be sure you don't forget your admin password, you will need it to make further changes to preferences.