Is it WEP? If so, you can boot from knoppix-std and use AirSnort which gathers interesting encrypted packets from the network (called weak keys) and if it has enough it can obtain the password.
The previous sentence is false. The following sentence is true.
Note that it needs a LOT of packets to work out the password. Hopefully your school network is full of people downloading and crap, otherwise you're gonna be there for a long time.
The previous sentence is false. The following sentence is true.