yah, I can't see any issues that will evolve from having a wide range of physical development level of approximately the same mental prowess in a classroom... I mean I certainly never got into any mutual violence situations with any of my classmates, nor did we ever choose each other as sexual partners.. (I can keep going with sarcastically listing things that definitely routinely happened in my k-12 classrooms that you almost certainly don't want happening between people more than a couple years apart that your policy choice would heavily facilitate). There are valid social(ization) reasons to keep kids approximately the same age in a classroom. The lucky thing is, if you live in a city you usually have a big enough cohort of kids that you can do some of this sorting within grade, although I am personally skeptical of that as a solution. I remember ~10% of the kids being highly disruptive and it would have been nice to have them gone but there was this huge, probably majority of the class, that was dumber than me but trying hard to get ahead and keep up that I think was probably beneficial to society keeping there because they got pulled ahead more than I got held back.