Why does my school block everything on the computer?

[Light]

I know why they have the parental blocks, I don't have a problem with them blocking Facebook, Twitter, Myspace or any other sites, and I could see why, but what I have a problem with is when they block 95% or more sites I go to where I kinda need to search around for a school project, or when I need a picture for a power point. I can never get anything done on a school computer because 95% of the useful sites on the internet is all blocked, they're all blocked for stupid reasons. When I go to the site at home, I don't see any nudity, no swear words, the site is safe to go to, the site is not a social media site. So why do the schools block all the useful sites that have no meaning or reason as to why?

I know mainstream media could be a problem for this, but for god sakes, we're in high school, why block and censor every single thing? I can't even find a single picture for a power point I need. I hate it when Pandora, Grooveshark and any other streaming music site is blocked, so I can't listen to music while doing my work quietly. Usually music helps me focus because when we get in the computer lab everyone starts playing games and can get very distracting.
Yeah, I like Jimmy Page and Ozzy and Slash, if you can tell...