You'll have to poke around the options for the graphics card configuration program, and somewhere there should be an option that says something to the effect of letting you choose between having the display mirrored on the second screen (projector) and extended onto it. You want to have it mirror the primary display.
You also probably want to track down everyone who's used the system recently, and make it clear they're not to mess with the settings like that. I'd suggest an email sent to everyone who would have access to the system. Doesn't single anyone out, and gets the message across to everyone at the same time. You may also want to create a limited user account that is unable to make changes to settings like that, and then make sure only you and any of your direct subordinates have the admin account password, with instructions NOT to give it out to anyone.
I'm a student at a school, and we're experiencing technical difficulties. Apparently, one of our school laptops is hooked up to a projector. Now, the problem is that for some reason, our projector thinks that it is the second monitor. I was wondering if there was a way just end that, and bring it back to the way it's supposed to be.