I agree with you in some points.
I realy recommend not to play ANYTHING at school. LOL, have I've gone made, or forgot my roots? Nope, I haven't forgotten I've started to play FPS when I was 14, and it was in the firs year of grammar school. But I can remember, that my grades were starting to become wors in that time too. Not much, but the change was significant. So I realy recommend, NOT to play any games in school (on the other hand, I just can't forget all those awesome deathmatches with quake2...

)
But If you can't stand, here's what to do:
Originating from the nature of quake games, you don't even have to install it on the pc you wanna play. Simply configure up the game for your liking, (and ofc. tune the graphical settings to the pc you wanna play on), and then write out the whole Nexuiz folder to a CD-RW (or a pendrive, that way it will be writable too). All the game data will be loaded in run time, and apart from the first few minutes, and the relative longer loading times, you'll never notice you're playing from CD. And if your teacher asks, you can simply quit and eject the CD or unmount the pendrive.