laws are not black and white, may be freedom of information act or whatever but does not mean they all must listen to it. just like our own government still has a crap load of top secret stuff they refuse to let the public know about, face it thats fact. a school can block whatever they deem they need to and there is nothing you can do about it, especially if under age.
you walk into a school and you fall under THEIR rules, if i could remember exact wording i would quote you word for word on exactly what i was told by a teacher, but its been a bit to long for me to do that atm. so a school DOES have every right to block any websites they need to, so that they can keep students off sites with material not appropriate for the school or to keep students off sites that can cause disruptions while in class.
should try reading the booklets some schools hand out every year, they clearly state what the rules are and what you can and can not do and not following said rules can get you into trouble even if some rules go against the laws you claim we have, because while in schools those laws sometimes do not apply.