pei: try telnetting to router4.limewire.com port 6349. If that succeeds, your school is probably only blocking port 6346. If it fails, they're probably blocking all non-web traffic.

In the former case, you may still be able to use Gnutella, but with great difficulty since most vendors use port 6346 by default. (BearShare is a notable exception.) In the latter case, you're really stuck without the sysadmin's help.