Ideally, each child would have the right to be in a class room where the majority were within a standard deviation of the child's ability and majority middle class by SES, Not everyone! Which would give schools the possibility to not compose classes with majorities below 100 or living in poverty, both extremely unhealthy classroom environments for everyone. In the highest reaches of ability,the slices as it were could get thinner, with Hg+ classes at the top, necessitating some concentrating and travelling for these groups, but precluding the necessity to bother about whether concentration of high SES kids in these classrooms distorts the distribution for the other classrooms, as the critical mass of middle class kids will be reached regardless.
Sorry, somewhat OT, pet topic of mine.
In our school district, the majority of public school students are at or near the poverty level, and many school districts in the US are like this. I know you're in Europe, but you're comments make it obvious you are very unfamiliar with the demographics of the US public school population, because obviously you cannot possibly make every classroom "majority middle class" when the overwhelming majority of students are poor. There has been a mass exodus from the public school system for anyone who can escape. To get them to return they need to find the classrooms and schools and surroundings to be acceptable places to put their children, in particular they should be safe with few discipline problems, and they need to have decent educational standards. This necessitates abandoning any notion of SES and other quotas. In particular, gifted classes should be based on ability/achievement and if the gifted population differs demographically from the general population, then so be it. There are no legitimate grounds for regarding that demographic difference as being any kind of problem.
