In addition, many schools do a "blanket" blocking of websites they deem inappropriate for school use, and sometimes, this tends to block sites that are perfectly reasonable in an educational setting.
The first thing you need to do is talk to a school official to see if the block can be lifted from a specific website. School officials usually will work with you, if the website has legitimate educational purposes.
However, if this option is not available, you can unblock blocked sites with a few methods that are safe, cause no harm to the host computer, and (most likely) will not get you into trouble. NOTE: Use these methods at your own risk.
Again, the best bet is to talk to a school official to get a site you need to get to unblocked. There are many sites that while perfectly educational are blocked right along with so-called "naughty" sites, and most of the time, schools and universities are willing to work with you.