Good point. This does imply that the teacher is watching the monitor rather than helping another student and then actively intervenes. So it is likely a different dynamic than a classroom where a teacher is observing the whole class at once and can take a somewhat more subtle action (i.e. speak louder in a direction of a student who is falling asleep). Khan academy may well be much more effective. It would be great to see studies that make clear comparisons of where it is most effective.