Will look into revertable vm's, I knew about snapshots but didn't know they could be set to not retain changes!
As for deepfreeze, since these machines are not on a domain and students have full admin rights, they could pretty much remove anything they want, including the deep freeze agent etc
In an ideal situation it would be great if there was a solution that worked in the lines of...
A deployment server of some sort hooked up directly to the classroom switch that can be enabled remotely...and then the engineer could potentially schedule a reboot and have the image dropped