Try the gnutella test:

telnet router4.limewire.com.6346

This should report "Gnutella OK".

[Sorry - I don't know how to initiate a telnet session using MACs].

There is a good chance that your school network has a firewall that is blocking gnutella connections. If the above telnet test works, the gnutella network /is/ available and you have some other problem.

Mark