So you want to shut the system down, but you also want nobody to know it was you. Let's see....
Open your cmd (Command Prompt)
In there type "net user (NAME) (PASSWORD) /add" without the " and () of course. This will create a new account. I recommend naming it something unrelated to you, such as Anonymous.
Give the newly created account admin powers. Do this by opening cmd and typing in
"net localgroup administrators (NAME) /add" Once again, without the " or ()
Get on the new account, and if possible, a different computer. Open cmd and type in "shutdown -i"
Hit browse. Leave all fields blank and press find now. Move all of the computers into the box and press Ok.
Set the computers to shut down. No warning. Set the reason to anything that is planned. I usually use Hardware Maintenance(planned).
Press Ok and watch the fireworks. It works like a charm at my High school, but I have never shutdown the entire network.
Good luck.
Now give the new account