The only site i have come across that it does not block is These forums. when i had my Xenforo forums up it would block it. I wish i new why, so i could edit templates to make it match lol.
Anyways my school blocks every site.... Unless they have a SSL then they can not (it kind of uses it's own proxy, as all sites are sent to a server before being passed on to the students.) Teahcers are like "they should block https://facebook.com" i say "They can not, the info is encrypeted and sent from the browser to facebook." lol teachers are dumbfounded when i say that.
Also the school uses one password for all there sub accounts.