I agree with you about the different groups, but those days are over in public schools. "Tracking" is illegal in American public schools. (ETA- but the European and Asian systems still use vocational and college bound tracking as their norms.) Every classroom has to have a balance of over achievers, average achievers, and under achievers, males and females, and the behavior problem kids are widely distributed to many different classrooms, whenever possible.
The fantasy is a bell shaped curve for every class, at least at the elementary and middle school level. No one is allowed to achieve too fast, and the focus is making sure the average and high performing kids will perform well enough on standardized tests to mask the effects of the behavior problems and underperformers on the whole educational process. And everyone singing



ba yah about the wonderful benefits of diversity, while overall academic performance declines, behavior management dominates instructional time, and instructional rigor is dismantled. IMO, behavior management problems are the single biggest obstacle to improving public education and student achievement today.
I will say one thing more, and then get back on topic. It's a complete fantasy to think that any social institution, like public education, can "fix" the problems that produce underachievement in 6-7 hours a day. It's a complete fantasy, IMO, to also think that busing kids from failing neighborhoods and failing schools will fix the root problems, either.
We simply can't "fix" the dire social problems where most the underachieving kids come from, and they go back to at the end of every school day. We can throw all the money at underperforming schools we want, but "we" can't fix the social circumstances that lead to student failure and underachievement, on a wide scale, IMO. Individual families and kids can personally work on closing the achievement gap, but society cannot fix the root problems-- poverty, crime, substance abuse, lack of parental education, lack of ambition, children who are not being parented effectively, or parented at all, etc. It's ridiculous to expect any school can fix all that in 6-7 hours a day, or even more ridiculous, for people to think that none of that should affect student performance.