There are so many variables though. "Private/Independent" schools, if good, offer so so much more than just the 'education', its the opportunities, the connections, the confidence to name just a few things. However, there are few grammar schools in a lot of areas, those that do have them have such stringent entry requirements so theyre getting the "best of the best" so it is no wonder they get decent grades. There are some outstanding state schools in London for example who out do most of the country but equally they have some decent kids, they're not the ones who have zero ambitions.
In our local area the local state schools excitedly advertised 70% of kids got 9-4 in their GCSE, meaning a while 30% didnt.... 15% got 9-7 (which is good) but the local Independent school advertised 58% 9-8.....
So whatever the media and society keep telling us, there are differences, private schools are not going to cease to exist, especially in our or our children's life times. I do think as a whole there will be a bigger difference between education levels. Its a postcode lottery, like the healthcare system is.