Comment by StableAlkyne

> should be a natural way to work on your own pace at your own level

Analogous to the old one-room-school model where one teacher taught all grade levels and students generally worked from textbooks. There were issues with it stemming from specialization (e.g., teaching 1st grade is different than teaching 12th). They were also largely in rural areas and generally had poor facilities.

The main barrier in the US to track separation is manpower. Public School teachers are underpaid and treated like shit, and schools don't get enough funding which further reduces the number of teachers.

Teachers just don't have the time in the US to do multiple tracks in the classroom.