I don't think it's inconceivable. But it's hard, and time-consuming, and in most cases, children not in the "middle" won't be actively taught as well or as often. DS is being heavily differentiated for but not so much...taught, right now...not at his level, I mean (except in GT pull-out, maybe). Still, thinking of it as inconceivable suggests we should simply give up on educating children of different abilities in the same classroom, and I don't think we should. That said, some kids do need to be skipped or put in different environments.