Yes its a prank.. not a ghost .

One of the other students (more than likely its the teacher) has installed a program like netbus, which opens up a back door and allows anyone that knows to look for it, once they are connected to see the screen, open programs, type, move the mouse, open cd rom etc... It could be remote desktop, vnc, pretty much anything that allows a remote connection and provides a video output. Netbus is probably the most obvious choice as its real small.


http://en.wikipedia.org/wiki/NetBus


Enjoy