If you lived in a semi rural area there were quite a few working class children at the grammar school. In the more commuter areas of the county, fewer children would be working class. My mother was truly working class and went to a grammar school. Many children in my year at school had parents with very ordinary jobs, road maintenance foreman, publican, shop worker, farm worker, and there were sons and daughters of solicitors, vets and doctors too.
One of the reasons why quite ordinary people, as defined by their jobs, had intelligent children was largely because they had been under-educated. Many parents had not had the chance to shine academically but were, nonetheless, intelligent. In the pre-war generation, when many of my friends' parents were born, there were grammar schools, but often the school teacher chose whether you went in for the 11+ or not. There is also plenty of evidence that some parents did not let their children attend the grammar school, even if they passed to go. This was often due to being seen as "getting above yourself" or because they could not afford the uniform. Even in 1966, this amounted to 3 weeks worth of my Dad's wages! My Mother and Father went to grammar schools. My mother and most of her friends were ordinary people and hardly any of them went on to university. Most had to leave at 16 and get a job to supplement the family income and this was far more likely to be the girls. My grandparents did not value my mother's intelligence and made her leave home and live in a nurses' hostel at 16, first as a nursing auxiliary and then as a trainee Nurse. She qualified in London during WW2. There is a ridiculous notion that the grammar schools were a universal passport to a brilliant career. They were for some, but definitely not for others.
It is clear there have always been middle class children at grammar schools, but where poorer parents valued education, maybe had under-achieved themselves, but had produced quite bright children, there was a chance of the grammar school for them. The secondary modern schools, however, were pretty bad and I can truly see the merits of comprehensive schools because so many children were failed in the secondary modern schools. Unless you are old enough to know what a truly awful secondary school was like, you will never understand why comprehensive schools were such a flagship policy for Labour in the 70's and, incidentally, were not changed back to grammar schools by MrsThatcher in the late 1970s. History lesson over!!!