If you do things to school computers, be paranoid about making sure the Administrators don't know who did it. If you think they're getting suspicious, I suggest dropping an anonymous letter in the office saying that someone you hate did it.
You shouldn't use a virus on school computers, because the principal or superintendent might call the cops over it. Fake viruses are good, and so are pointless messages that make it look like you're getting hacked. For example, use the code below in a .vbs file to make an error message.
Code:
X=msgbox("A virus has been detected on the computer! Please restart your computer."vbCritical+vbOKONLY,"Windows Firewall")