Playing pranks on a computer at school is easy and fun, even if you don't have much technical knowledge. Most of these pranks take just a few seconds to pull off, and they will have some hilarious results.
One of the most effective computer pranks involves swapping out the Internet shortcut with the computer shutdown shortcut. Right click the icon for the Internet browser (most likely IE, Firefox, or Chrome) and click properties. Replace what's in the Target box with "%windir%\system32\shutdown.exe -r -t 00" and click OK. When your friend goes to open the browser, the computer will automatically shut down.