Well I went to a girls grammar which turned co-ed and comprehensive part way, so it ended up not quite knowing what it was supposed to be for my last 4 years there. It didn't make much difference for the sixth form, because you had to have 5 O levels to get in, so that bumbled on, much as it had done before. And I do mean bumble. In those days, you only needed 5 O levels to go into teaching, so it was full of girls marking time.
There are no grammar schools where I live, and I wouldn't chose to live in a grammar county. Of my two DC the one who has done best academically is the one who might well have not passed the 11+ being good at maths, but not so good at English.
The 11+ also favoured the child who was quick - so the slow painstaking child often missed out. Yet being slow and painstaking can very much be an asset in later life.
Yes, you did get the nice middle class children in the top sets in their Comprehensives, but they weren't top sets for all subjects, and similarly there were very few children who were always in the bottom set. Most children had a chance to succeed at something.