I don't know if anyone is still looking at this thread, but if you're going to do a remote shutdown with XP, I've noticed that a certain security setting has to be in place.

Go to Start > Programs > Administrative Tools (provided that it's there) > Local Security Policy

In the little tree to the left, make sure you go to the Security Settings > Local Policies > Security Options menu. Then scroll down to where you see "Network Access: Sharing and security model for local accounts", and set it to Classic mode as opposed to the Guest only mode.

This allows any local users to log on remotely, then you can use a tool like <a href="http://www.sysinternals.com/ntw2k/freeware/psshutdown.shtml">psShutdown</a> to log in as that user and shut 'em down.

Granted you'll need a local admin account on that machine and know the password, so that might not work at your school, but it's useful to know for other purposes.

If nothing else, while you're at school, try a command like this:
net send * Help, the principal touched me like Michael Jackson!
That's always fun (we got in trouble back in school for playing with the old winpopup on some Win*5 machines).
Now, the only problem is that there was a vulnerability in the messenger service (which delivers the net send commands), and Microsoft solved the vulnerability by sending out a patch that disabled the serivce by default. However, your school might have the service on so they can send out outage notices or what not. Either way, it's worth a shot. Oh, and btw, any message you send out will have the host name on it that sent it, so do it from your friend's computer.