In high school, we had laptops provided by the school and imaged by the IT department, who had logged into them. All we had to do was use a tool to recover the Windows password (yeah I know, skiddie, etc.) and we could login as IT admins on any computer on the domain.
Was pretty innocent about the whole thing, changed a few backgrounds remotely, and sent messages and shutdown people's computers in class remotely. Unfortunately I changed a default Windows background image, which meant it showed up something unsavoury for everyone who logged in, and got caught, copped detention and a many, many page letter about how I shouldn't do it.
I too changed the default Windows background. However it was just the default image with an almost transparent eye in the corner. It lasted a few weeks before it was reverted one day.
Was pretty innocent about the whole thing, changed a few backgrounds remotely, and sent messages and shutdown people's computers in class remotely. Unfortunately I changed a default Windows background image, which meant it showed up something unsavoury for everyone who logged in, and got caught, copped detention and a many, many page letter about how I shouldn't do it.