hmm well maybe a mac filter? the first parts of a mac reveal the manufacturer of the wlan chip, if I had to stop ITs from using my wlan and I were ignorant of macchanger, I would choose this way.
As I know macchanger, however, you might give it a try:
Get the MAC of some wlan card (maybe the one of your laptop) and write it down.
When you are at school, change your ITs MAC to the one your laptop has and see if it works.
Remember, however, to change it back when you are at home, as you shouldn't have two computers with the same MAC in the same (w)lan.

(Or: take your laptop's mac and change the last pair of hex digits)

Otherwise I see no way of removing ITs from a network with simple means (no vpn and so on)