I can really relate to mb's post. Looking at my oldest son, he is very much affected by the general standard and expectations of his peers and teachers. He moved primary schools three times, doing much better at the school where expectations were the highest. He is still easily influenced by disruptive behaviour. Add his generally laid back/lax attitude to work and I am sure his level of work would really suffer if he went to the sort of school HRH talks about.
His head teacher (state primary) even took me to one side when ds was in year 5 to urge me to aim for a 'good' secondary school for him - by implication grammar or private - as he is so easily led.
That made me think, as in our borough, the comprehensive system rules and there are no grammar schools, so the head teacher was really not towing the party line when he had his private talk to me.
As I live in a London borough with low secondary school results, I am very used to talking to parents who send their children to secondary schools well below average in the league tables. There is little choice if you stay in the borough, unless you are catholic (the catholic comp schools have better results). My ds goes to a grammar school outside the borough.
I know lots of parents happy in their choice of borough comp, once their child starts school in year 7 - they seem pleasantly surprised by the ethos, the teaching and their child's progress in year 7 and 8. Their children seem happy, too.
However I have known some very unhappy parents of older, possibly more rebellious children - once their child gets to studying (or not) for GCSEs. As I don't know as many parents of older children, I am less sure how widespread this is. And actually, the most worried parent has a son at grammar school.
The youth drama class my son attends has lots of 12 - 16 year olds from the local comprehensive schools - they seem like a really nice, hard-working bunch of individuals.
Sorry, not an answer to your question, HRH, just some random observations.