in netconfig choose dhcp as means of obtaining IP. Your school has a dhcp server with static leases for registered MAC's - sort of a security measure I'd guess, not registered MAC won't get an IP, or it will get an IP from a range that isn't allowed by your compus' firewall to go to the net. Just remember that you will have to reconfigure every time you want to use your laptop at home and school.
P.S. You will have to issue
every time you reconfigure so that you don't have to reboot