And yet, on the other side of the contexualisation debate. My DS did his GCSEs at his local comprehensive and bailed out to a highly selective independent school for A Levels (which required an entrance test, and an interview, and then to meet their GCSE requirements. We discovered afterwards that DS seems to be the only state school candidate who ended up going there).
My DS's GCSEs look pretty impressive compared with his cohort, and definitely less impressive compared with those he has now joined at the flash independent school. On of the things that discombulated him a bit at the beginning though, was to be in classes where everybody is bright. He is doing well there, but he is not at the top of the tree. He is now seeing people in his flash independent school being turned down for interview at Oxbridge, people he is very confident are streets ahead of him, not just due to teaching, or opportunities, but simply in terms of ability, motivation etc, while he is seeing people at his old school getting interview offers, people he felt he was streets ahead of in all of these things, but who stand out because there are all the more average kids in the comprehensive, who would never have got steller GCSEs.
And it is odd. Had he stayed at his old comprehensive, he might well have applied for Oxbridge, with the handful of others who are (it is just a handful). At his fancy independent school, he felt it was less appropriate, especially as he had decided he really, really wants to do medicine, whch is hard enough, and so decided late last year not to try (which is why I am still lurking, because it was a path not travelled). Admittedly his comprehensive is regarded as a very good one, has an outstanding rating (although from ages ago), and probably mostly sends maybe, one, to Oxbridge a year (as opposed to his independent school, which can send dozens). So it is not comparable to a comprehensive taking refugee children from sink estates. But as someone who has now at least somewhat seen "both sides" now, we are really not sure that the contexualisation does the job it is supposed to do.