This is an easy and powerful tool.

If you have a blank CD, an ISO burning program, and a school with Windows XP/Vista on its computers, then congrats! You can really fuck with a school.

First off, download this and if you don't have an ISO burner, I suggest FreeISOBurner.
Once you have burned the ISO onto a disc, you're half way done. Restart you're school computer and as it is loading, tap F12 a few times. You will hear a beep and the computer will take you to your boot options. Pop the CD into the CD drive and choose to boot from the CD Drive. You will see a bunch of shit pop up. Just tap enter when you see it stops loading directories and such, (All the words and characters and shit.) and the program will guide you with the rest. Once you finish, make sure to hit "q" to quit and type "y" to save your work.

Now you're probably wondering, 'What the fuck did I just do?' Well, here's your answer: On the log in screen, put your username as "Administrator" and click the drop box and choose "[Computername]- This Computer" and hit enter. Voila! You have access to the computer's (not network's) Admin account. Now you can install shit and not have any restrictions. (Websites are still blocked though.)