This is kinda how Montessori schools do it - at least the one here where my partner teaches. They aren't online, but the student gets to chose to some degree w

This is kinda how Montessori schools do it - at least the one here where my partner teaches. They aren't online, but the student gets to chose to some degree what they are working on, however they can only move onto the next level of a subject once they've passed the 'works' that come before it. The grading of a work involves marking problems correct and incorrect. Note that this is 1st - 3rd grades