dstiles - 6:40 pm on May 23, 2013 (gmt 0)
They're schools. With kids aka juvenile hackers! :)
The proxies can vary within the IP group depending on the individual school's setup, though they are usually different versions of the same basic proxy server.
The proxies have to be genuine, known ones for me to whitelist them, but what goes on behind them I have no idea and little chance of finding out.
I did once have a long email conversation with one of the providers and we ironed out a few of his problems but on the whole I have to rely (now) on backup blocking techniques (Oh! A google bot. How novel and unexpected!).