Blank Screen on VNC sever computer

I have installed tightvnc on about 150 computers last summer with great success, I have tried to load it on five this year and when I go to view or interact with the computers all I get is a sreen showing that I am connected to the computer but ther are no icons or start button or anything usable from the other computer, I can use remote desktop to the computer but that disconnects the other user. I use this at a school district so I need to be able to monitor students without them knowing it. Help