Just keep trying passwords until the heat death of the universe, you die or graduate high school.

If they use WEP (does anyone anymore) then there are plenty of programs floating around to help you sniff the key, otherwise you're probably best off figuring out where it's stored in the registry and grabbing it from there. Of course, regedit is probably disabled, so just start guessing or give up and realize there's a reason they don't want you on their network.