[petercooper]

you'd be able to measure an effect in the places in the UK where they were retained.

I live in a very rural area where a grammar school has been maintained (which my wife attended) and most of the people who go there end up at good universities and then move away for good careers in distant cities. Meanwhile, the people at the comprehensives usually end up staying in our area on lower paid jobs.

Does this lead to be a better society? It certainly provides a way for poorer but smarter people to find a way 'out' of a humdrum rural location, but I don't think it makes our area any better in and of itself.