Device to display any copmuters video output on 1 central TV screen

I’m wondering if anyone knows of a device that would sit ona a network and be able to get the video out of any computer on the network, and display that video on a large screen TV?

Used for training in an office, and being able to call up any computers screen and display it on a big tv that everyone can see.

If nothing exists, I suppose I could get a video card with TV-out for the instructor, have them run a VNC type program so they could monitor the computers on the network, and then they could select one computer to display on the tv from the video out..