What you want to do is chain the proxies and make a connection through the chain. In most cases the most appropriate program to do so is:
http://www.htthost.com/httport3snfm.exe
What you do is input the settings for your schools proxy, then tell it to use a proxy on port 80 for external use.
If you have it setup right you endup with a listenning port on local host which you can point your Browser, IRC client, or pretty much whatever you want through.
Otherwise you might like to use a CGI Proxy, CGI::IRC or WebMessenger depending on your needs.