If you're in a group and some of your peers leave the group because they've mastered the material, you might feel like you're being 'held back'. And observers m

If you're in a group and some of your peers leave the group because they've mastered the material, you might feel like you're being 'held back'. And observers might feel that certain demographic or identity groups are being 'held back' at a higher rate than others. Sure, perhaps you leave it up to the student to decide which group they should be in. But what if they move to the next group when they're not ready? Does the tutor cater to them or to the rest of the group? What if some demographic group is less confident on average, and doesn't move ahead even when ready? The setup you've described might work, but people will find all sorts of reasons to be mad about it, to claim discrimination and/or to use whatever outcomes there are to claim that somebody is being treated unfairly. The academic literature on 'mismatch' doesn't all lean in one direction. You should read Sonja Starr's article on the coming magnet school wars. She definitely has an opinion, but she made a decent attempt to present fairly both sides of each issue she covers.