Interesting. I guess the reality is that once a school gets a reputation of being 'good' already there's a good chance 'the dynamic' will change and it can become self perpetuating, as in, if a family is looking to move to an area, they might actively choose to move into the catchment of a school that's 'good' or evidently improving. In this way, house prices might rise, therefore a form of 'selection' arises. Or maybe less confident parents shy away from the school fearing being 'out-classed', not-for-the-likes-of-us. Or the school's discipline policy may be beefed up, safe in the knowledge that for every 'little oik' they expel as being too much trouble to guide and reform, a Felix or Charlotte is queuing up to take the place.
And of course, I'd agree that 'a good school' should be one that (Q:) offers every child the ability to reach full potential and adds the best value to all- but in reality, there aren't many of that utopic ideal around, are there? Because, of course, schools get hampered by inadequate parenting- and they can 'offer' all they like, it has to be taken up with focused enthusiasm by the DC at the school. I read here on MN very recently in a thread how the excellent, gleaming facilities of a new school, maybe an academy?- were systematically trashed by the intake as the DC had no grounding or cultural understanding on how to recognise the value of such facilities to themselves and their futures, horse to water etc.
SO as a result, where advantage can be gained, parents 'choose' and schools 'select', one way or another.
I'd ready admit our comp selects by house-price, leafy, MC, highest results in the county. But it appears to do well by all its DC, which is why I chose it: DS1 would've been OK at any local comp where heads aren't routinely flushed down toilets, as he's A or B set-material; but I was concerned about my B-C-D level DS2 and how he'd do in lower sets in less disciplined schools. But, in fact, judging by his reports, DS2 is doing every bit as well as DS1 was at the same age, so the school is doing it's job with less able DC, too.