If you setup a server, as in the Mac OS X Server Installation, pages 23ff show configurations, you can have multiple users, but they must all have computers. Some installations can be done with dumb terminals but that is not what you seem to want. Unix machines do have a multiple simultaneous user capability but you still have to have a computer for each user, again, not what you seem to want to do. Right now there are two other users logged in on my Sun workstation on the desk behind me...they are logged in because I permit them to do so.
And last night for an experiment I connected my iMac and MBP by ethernet wire, logged in on the iMac, then logged in on the MBP and from there on the iMac...but I set up the network to permit that and then disabled it again. You can create security holes doing things like this.
As dwb points out switching from Apple to PC equipment is a false economy that lures in administrators. In our county school system we had a superintendent who hired a good friend...who had PC-industry ties. Decided to remove all Apple equipment from the elementary schools and switch to PCs. It wasn't long before they ended up going on a 4-year cycle of "refreshing" the equipment, i.e., completely replacing all hardware in every school on a four year cycle. Since my wife was a Media Specialist, aka librarian, and responsible for technology in her school, every four years she had to develop a complete school map of where each teacher wanted the computers in their classrooms, complicated by where the technicians predetermined the locations of the network wire drops, of course. then teachers get shuffled over the summer and all the plans are for naught.
Anyway, there really isn't a solution to your problem except to buy a large number of computers and let each student use one and have them networked to a server, print server, printers, etc. This is not inexpensive and cutting corners will only cause headaches for you.
