I'm in charge of our school proxy server.
I don't bloc anything xcept the stuff the gov says has to be blocked (and I don't have any control over that anyway - the ISP does that). BUT
the pressure from teachers to block stuff at the school level is huge cause kids read Farkin and other stuff rather than doing what they are meant to be doing.
If you want to be able to read myspace, farkin and stuff at school, then do your bloody work when you are meant to and read the other stuff in free time. That gets teachers off my back and I can leave the net open.
I have a "don't ban it until it becomes a real problem" attitude. Trouble is, some little shits make it a real problem! If they just did the work and got it over with, they would probably have time to catch up on the forums and the teacher wouldn't give a rats if the work was done and you asked nicely!