Most schools enable a firewall to restrict you from visiting inappropriate websites, images or other material. Unfortunately sometimes they block perfectly legitimate stuff, which sometimes you need to access for a school assignment. In these cases, here are a couple tips for bypassing your schools internet security.
Try putting "https://" before the web address, instead of just http://, this works a lot of the time if the website supports it.
If your school has not already blocked it, you can go to www.sefreak.com and sign in to the portal using the username student and the password random1 to gain access to a really quick proxy.
If your school frequently updates the block-list, and you want to always stay informed about the latest proxy websites, you can sign up for a mailing list here: http://www.peacefire.org/circumventor/.
If you have access to your own computer, consider computer programs such as UltraSurf or HotSpot Shield which help get around internet censorship.
For the more technologically-inclined, you can set up your own proxy server using a running computer connected to the internet at home, or using Google Apps: http://lifehacker.com/5484934/run-your-own-free-proxy-through-the-google-app-engine
Things You'll Need
Only use these techniques if you are willing to accept the consequences of getting caught.