There's pros and cons to any style of teaching, but the Montessori method has some of what you're looking for. Specifically, it allows students to move as fast or slow as they want in any particular subject area.
I went to a Montessori school for a year in middle school, and I remember being in a class with 5th graders doing math at the 9th grade level. I also remember a few kids in 5th grade doing math at a 4th grade level. The key was that the teachers could teach their subject at all of those levels.
I feel like some of this could be implemented in a less structured educational system. Being able to move at varying different paces while still socializing with peers of the same age is a good combination. I would add a little more structure than Montessori schools though, to make sure that students are moving at least at a baseline pace. It also takes some scheduling work to make sure that teachers aren't teaching an hour lesson to one student at a time.