Doesn't the assumption that these bright and/or middle class children will 'massage the statistics' whilst having no real effect on the others depend, in fact, on the assumption that a bright child will indeed 'do well wherever he or she goes', though?
In fact, I think it is not so much that four or five children from wealthier/more academic/more aspirational (take your pick) backgrounds will improve a school just by being there, than that the drip drip effect of any family who can afford to opt out either by buying a house elsewhere or going private has a very detrimental effect on the shunned school, in terms of ethos and actual sustainability.
The whole idea of 'ethos of success and achievement' which many value in private schools is at least in part due to the kinds of children who go there. If, in a struggling school, you simply do not have the numbers to make up a convincing and effective 'top set', you cannot hope to have that ethos.
If parents of bright well-behaved children opt out in the belief that those children would be bullied for achieving, this sends a powerful message to the children on both sides of that decision: 'you would be bullied by children like that' and 'you will bully anyone who is clever, so best for you never to be exposed to such children'.
A case in point here is a school in my city which is currently facing closure due to low uptake of places. Parents of children who attend it are, from my experience at least, happy with the school. However, its catchment is made up of 1) a big council estate (usually cited on MN threads about this city as somewhere you don't want to end up near, although usually also the comment is made that it's nowhere near as bad as estates in other cities) and 2) one of the wealthiest areas in the city.
(we have a strange set up here whereby a lot of the wealthiest areas are in the catchments for the least desired state secondaries - not much buying for catchment here).
So in this area, parents have tended to get into the next school down the road on a 'foundation place' if they are savvy enough to wangle one, or else they go private ('well we felt we had to, as we're actually in the catchment for B' is a comment I've heard several times).
Thus, choice for the better off and the more savvy has resulted in no choice at all for the rest - to the extent, now, that their local school will be closed. This school is not a bad school: have never heard of any crime there, and I know that two of dd's favourite and best teachers went there at the end of last year, so the teaching cannot be all bad, to say the least. But it will go, because so many shunned it out of hand. The message to the children there has been negative for years (you wouldn't go there if you had any choice at all) and now it is one of utter failure.
I think this is wrong.