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.
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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
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Only use these techniques if you are willing to accept the consequences of getting caught.