This is a major weakness, but not a debilitating one. Teachers using the model have confirmed to me that they, too, find it possible to make workable estimates of students' stages. In our class, beginners always practice where they can see more advanced students doing the same thing. This modeling is highly effective. Perhaps schools should find ways of providing such modeling, instead of grouping children of the same age together.
Some students take longer than others, but failure is almost unheard-of.
Also, feedback in class is frequent, objective, and task-specific. As students advance, they begin to teach others. And while the practices are highly regimented, the higher forms encourage a good deal of individual freedom and initiative. Somehow, students gradually become more self-directing as they progress through this highly directed system. These are important components of self-direction.