Or you can hack into the main server or if they have poor security just browse through my computer until you find the files that block proxies.
At my school all I have to do is type in //msfileserv in 'My computer' and it takes me to everyones storage drives, including the Administrators. In one of the server drives there was a file named "Proxy Blacklist" and all I did was deleted it and then deleted
Deep Freeze and bam, no proxies blocked.
Or, if all else fails, you could try Brute Forcing the Administrators password.