When I was in grade school, we were in groups through the early grades. Now, it may have been the teacher...airplanes, jets and rockets. I had the same teacher for 3 years, grades 2 through 5 (accelerated through 4). And each group got material geared to their level. You could move groups, you were not stuck. But we were in a group with material geared to our level. What happened to that? You don't have to go with the obvious categories, but yellow, green and violet groups.