I DO NOT recommend doing this at school. First off, you can get yourself in to big crap if you do.
Second, Administrators block cmd.exe so you can't do stuff like this.(Easy to pass though. They already have a bunch of Instructables on how.)
Also, I dont know if this is considered hacking or not, so only do it to people who wouldn't care.
Now that I am done being Mr. Strict, the boring old normal teacher, lets get started!
Step 1: Steal the IP adress!
To shudown or whatever you want to do, you first need your victim's IP adress. This is the computer's connection to the Internet. The easist and fastest way to is to run cmd.exe (Command Prompt) on your victim's computer. Click start and then run. Type in cmd.exe and click the RUN button. Type in this afrer Command Prompt opens (NO QUOTES!):
Copy the IP down on a piece of paper.