Just going to wade into the debate to agree with OP, but I went to a state grammar school. However it was the least socially mobile school in the country - something like 6 kids on Free School Meals in as many years. I hated it so so much.
My first day was like the opener of Mean Girls - no-one talked to me or even looked at me. From then on, it was snide bullying - nasty comments, being ignored, online bullying. At my school was a complete hierarchy and social divide - not so much rich and poor, as we were very MC, but basically if you didn't wear Jack Wills and a short skirt, weren't sporty and didn't routinely perform sex acts on boys at house parties, then you were not cool. Sex and drug use were rife - people's parents would go away on holiday and they would have Skins-style house parties. It was a horrible place to be and I felt like I never belonged, I wasn't even "allowed" to sit with the "cool people" in my year group on the train home. I spent 4 years crying, self-harming and binge eating till I went to uni - where I went on to have the 4 best years of my life.
I was genuinely shocked that people smiled in the corridors, let anyone sit with them in lectures, and would talk to you even if they didn't previously know you. I loved uni so much and it was like a breath of fresh air, even though I went to a very MC uni. It was just different. People were NICE. Jack Wills wasn't required. You could have blue hair. Everyone was accepted and had a group for them, the groups weren't cliquey, people made all sorts of friends, there was no more hierarchy and I just felt like I could be myself. No-one cared if you were shit at sport or dressed in vintage clothes.
My partner went to a pretty good comp and said it was nothing like my school, he loved his. When we have kids, we won't live in a grammar school county. We'll just live in a town with a decent state school. I want my kids to actually mix with the rest of society (I didn't even know what a Free School Meal was till I read it in the news), and not be caught up in a toxic rich-kid culture where the only way to win is to be horrible.
I feel academically, it wasn't worth it - I got 2A* 2A at A-level, he got AAB, we got the same degree from different unis and are now in the exact same job.