For FPS games I recommend Noughts and Crosses.

If by FPS you mean First Person Scribbler.

Otherwise, bring stuff from home on floppies (that's what I did with UFO hehehe, took 4 floppies, though). Then bring UFO2000. Then you can play over the school's internet facilities.

Works a charm. On my computer I can't download anything at school, but I can copy and paste from floppies :D

You could also try Quake 1. It's small, simple, quick and worked over TCP. Although I'm not sure if it's abandonware or not. Probably not.

We used to do this at school all the time. My 15 minutes of fame came from setting up a dedicated server for Quake in the computer room. AoE 2 was another popular one. Before that it was Wacky wheels and Scorched Earth.

i brought in doom 2 one day at school and now every one plays it. we even have started are own clan.

If your school computers are really good and you have a lot of hard drive space for your account, then there is of course the Serious Sam tech demo, with co-op multiplayer on anything up to about 8 computers (I think) or even better, two on one computer.

Other games to try include Dark-Future, a really good text-based multiplayer thing. Also try Kingdom of Loathing and Project Rockstar (which might well be banned, it is rather good).