You are certainly correct in identifying one of the challenges to implementing this kind of model. But the reality is, that students in most classes are at diff

You are certainly correct in identifying one of the challenges to implementing this kind of model. But the reality is, that students in most classes are at different stages of understanding. We're just used to a model where everyone is expected to be at the same place all the time, and tend to bore the students who are "ahead" and frustrate the students who are "behind". There are other models, which address this issue of pacing and differentiation effectively. Sorry to not give a more complete answer at the moment.