I think a lot of it isn’t to do with private or grammar schooling, it’s having parents ‘in the know’. So, if your parents know about City careers, or medicine, for example, have friends they can ask for advice or introductions for you, you’re always going to miles ahead of the equally bright comprehensive kid who has no idea these kinds of path could be open to them.
Then add money - if your parents are well off you don’t need a part time job to fund your studies. A high proportion of my Russell Group university friends had flats bought for them as their student accommodation so they had a foot on the property ladder in the 90s and have made an absolute mint on that alone.
I had no idea about Oxbridge until the application dates had already passed. I hadn’t a clue what any of the companies on the Milk Round did or why I might apply. My friends all seemed to have a really clear plan, which I now realise had been there since A Level days.
DH and I are now doing well (both state educated, neither went to grammar) earning six figures each, but we were late bloomers as we had to work it all out for ourselves by grit and wit!
My kids will go to state all the way through, BUT will have the advantage of everything we learned. I really think that makes the biggest difference.