All Gnutella clients are able to share even behind a firewall. It's just better if you can open a port because then people can connect BACK to you and "push" a file to you (because they are also behind a firewall).

The "push" files are easier to get because those nodes are less busy, so it's really important to port forward on any firewall (computer and router).

In this situation, you may be blocked by some school router filtering, they may not be allowing connections to OUTGOING ports other than 80.

You can try to get connected to only IPs that also list a port other than 6346, if that works then only 6346 is blocked and so be happy with whatever you can get.

You can also point your browser to some of the IP & ports listed and some nodes like bearshare will respond just like a webserver and show a page, that will also tell you that you are getting through.

This could tell you they are filtering based on header content because they are letting browser connections through.

If they got "smart" and are blocking based on the content of your packets (looking at the header) then you need to share using something like "MUTE", it uses encryption to prevent that type of blocking.