Some of my favorite and easiest pranks to do are the computer ones. These can be done on any computer that you and other students have access to. (like the ones in a library or in the back of the classroom). I wouldn't recommend doing it in a computer class because the teacher knows how to fix the problem and they sometimes have key loggers installed to see what you type. You also have to sign into the OS using your name so they know who messed with the settings..
The library is easy to do if your quick because most schools have a program that will lock you out of the system if your not on task or your in a menu you shouldn't be in.
Screen Flipped Prank
This is the easiest prank to do because it takes less than two seconds to do.
All you need to do is hold down the following keys.
= Upside Down Screen
Do the same key combination except press the Up Arrow in order to return the screen to normal.
This usually works on Windows Xp.
I tried it on Windows Vista, but it doesn't seem to work for me.
Another easy prank is to replace the internet icon with the shutdown command.
There are many instructables on how to do this so I dont think I will really need to explain this in great detail. Basically you create a shortcut to shutdown and rename it internet explorer and use the internet explorer icon. People use the internet the most in school so it should shutdown the computer everytime they hit the fake icon.
This is very easy to do. All you need is a post-it note and stick that post-it note over the "eye" of the optical mouse. It will keep the person from moving the mouse because the "eye" can not see the changes in direction.
Assuming that the computer is a desktop at your school, you can carefully
pry off a couple of keys from the keyboard and switch them around. You can do it on a laptop computer, but the keys are fragile compared to the desktop so its not recommended.
BSOD Screen Saver
The Blue Screen Of Death basically means that your expensive computer is now an expensive paperweight. Microsoft actually has a screensaver that simulates what happens when the BSOD effects a computer. You can get this screensaver from http://technet.microsoft.com/en-us/sysinternals/bb897558.aspx
It will be funny to see the persons reaction when they find out they have a huge problem only to find out that its fake when they move the mouse.