Below are some options for maintaining your Mac lab. Get one type of computer (G3, G4, G5, iMac, MacPro etc.) installed the way you need, create a Disc Image then deploy to the same type of class computers.
You can push that image to each computer using Apple Remote Desktop (ARD) (via network) or if you don't have ARD (or a network), put the disk image on a Firewire Drive and clone the image to each computer.
The creator of NetRestore, Mike Bombich, has created some great free tools at his website for Mac lab management. MacEnterprise.org has a webseminar with an outline on how to use his freeware NetRestore with ADR. In the webinar, go to the "deployment options" chapter of web cast.
ADR and NetRestore are powerful tools because they allow you to clear all the student preferences and remaining work at the end of a term.