As mentioned in the paper, there are Khan Academy "coaching" tools for monitoring the students. The site allows you to monitor the relative progress of the students so if one started to slow down you could pick them up on that: http://www.khanacademy.org/images/class-report-medium.png
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.
