Doesn't Macintosh Manager take care of this on OS X Server?? I remember there being something for Mac OS Classic called Apple Remote Administration Toolkit or something of that nature, that allowed you to view others desktops (you could tell you were being monitored because you would see a pair of eyeballs on the menubar) as well as remotely control and lock out client machines.
We used to use it in our MacLab for training seminars. It worked great when presenters needed everyone's attention or when they needed to monitor what others were doing.