"Can we stop repeating the myth that that children from certain families will do well in any school? "
I agree. I really wish that were true, but I think it's a bit naïve to say that really. However good a teacher someone is, there's only so much they can do to meet the individual needs of 30 children of wide-ranging abilities and differing challenges, in circumstances where dwindling financial resources mean you have no TA support.
DS1 is the kind of child who will knuckle down anywhere, work hard, not be distracted and did really well in his state primary.
DS2 is equally bright, probably more so actually. But he is very easily distracted, does as little work as he can get away with, rushes through things as quickly as possible. He spent a fair proportion of last year reading his books in class because he'd rushed through both the normal and the greater depth work available and had to occupy himself while the rest of the class caught up. There was no TA resource and no extension work. He was bored a lot of the time and started to disengage.
By contrast, when DS1 was in the same class (with the same teachers) 3 years previously, he worked hard whether someone was giving him individual attention or not, and also there was TA resource available, so he was taken out of class a fair bit for extension work.
The fact that DS2 now has 11 children in his class plus plentiful resources at school for individual extension work to meet his needs is already making a huge difference. He is much more engaged with his learning, is doing lots of extension stuff and is making brilliant progress.
It's just naïve to say that intelligent children from supportive backgrounds will do well anywhere. DS2 would probably do perfectly ok results-wise if he had stayed in the state system, and that would have been my preference. But it is very, very clear to me that he will do hugely better where he now is. It makes me sad that that is the case, but as I say, there is only so much that can be done in circumstances where in real terms, state school funding is being cut year-on-year.