Is censoring the internet legall?

is censorship of the internet just? the internet is supposed to be the stage of free speech and expression free from censorship or opression yet almost every school in australia runs a very fascistic internet filter preventing students from visiting almost anything they would find interesting, youtube facebook myspace music and mivie fan sites are all blocked, and while it might seem like a good idea, in terms of minimising distractions, the people who conceived the idea of filterng the internet at schools are never actually in a clroom, students have almost no opportunity to use the internet recreationally during cl time, they can however use it in this way during free periods recess lunchtime before and after school, however these filters are still in place at these times impeding the use of the internet, personally i have found that sometimes thes rudimentary filters even hinder studying, as they block sites using a crude and fatally flawed key word system, my question is i guess, why is it neccessary to filter the internet if adequate supervision and internet saftey practices are taught it eliminates the need for such filters, furthermore is it a just thing to do ? what if the government decides to censor website of its political opposition or censor website harbouring alternate views on things? when does the censorship stop? and is it even legall?