If it's gamesurge, check out the server list, there's a few listening on non-standard ports.
http://gamesurge.net/servers/There's also the option of getting the encrypted password list, and cracking it for the admin password...
Or booting say, knoppix, renaming files for the firewall, and watching it disappear..
Or SSH tunnel through another computer (what I do),
Also, if the school doesn't keep up on their patches, well, thats just asking for it. There are a couple exploits that can get you admin level shell access.
Or boot off a knoppix disc, and IRC from that.
It all depends where the actual port filtering is being done. If its computer-level, there's a bunch of ways around it.. If its at the actual connection, your only real options are non-standard ports, or some kind of encrypted tunnel.
On a side note, MAC address filtering is useless, as I proved to my school.