https://addons.mozilla.org/firefox/2278/Most of you probably know about this, but I'm posting this for those who don't know.
Ghostfox is an add-on for firefox, which is just perfect for surfing at school without getting cought.
The only thing you need to do is move the mouse from the right to the left and ghostfox appears in whatever program you're using (Word, Excel, but also Explorer and your desktop). When a teacher comes close you quickly move the mouse to the top/bottom and ghostfox dissapears (it's still there, but you can't see it).