> First, you need to teach to the level per activity.

This can be accomplished.

Imagine you have a set of similar students by age, say Kindergarten - 2nd grade. Imagine that you have 4 classrooms and 4 teachers, each with a particular subject (math, science, reading/writing, geography/history). Each teacher teaches the same subject at multiple levels. Then you just send the student to the skill appropriate class.

If you were to take classes above your age determined grade, it'd basically be like skipping a grade. If you're at your age level for learning, there's no difference. But for everyone in between they get a mix of more advanced learning where they need it.

I may or may not have had first hand experience with this model