I do think there is an issue about the schools in what is a predominantly white part of Surrey being taken over by children from ethnic minorities though. It does to some extent skew the character of a school which essentially should be serving the local population. It also means that many of the children don't have a natural allegiance to the school or the area.
When DS and I looked round his super-selective, the mum of one of his school friends (the children are first generation British) commented that there weren't enough children from ethnic minorities and she didn't like it for that reason. But by the time DS started a year down the line, two of his four form entry classes are 45 - 50% children who have English as a second language.... They are certainly very focused on the sciences if not on English!
I have heard it said on more than one occasion and from a variety of individuals, that a lot of the children from ethnic minority backgrounds (and their families) do not fully integrate in school life - they get the school the excellent results but I think the whole point is that the Heads and teachers want boys who are not just academic automatons but give 'added value' to the school (and vice versa) too.
It has to be said though that DS's English skills are lamentable on occasion and we are an educated family with DW speaking RP! And we have so many books that half are stored in the attic!
It is true that in DS's year one of his classmates who go into one of the girls super-selectives 'came out of nowhere' and surprised the whole class (and parents) in doing so. She had always been a middle-table in a clever class pupil. But three years of tutoring worked magic.
Before I am shot down, let it be known that I was in essence one of the white working class boys who now sit at the bottom of the academic attainment league tables....