When I was growing up, there was one large secondary modern school in our area and two grammar schools for girls and two grammar schools for boys. We sat the 11 + and, if we passed, we could choose the grammar school that we wished to attend. If we failed, we went to the secondary modern (SM) school.
The 11 + was supposed to screen children onto either academic or vocational pathways. If a child passed the assessment, s/he went to the grammar school to be nurtured towards university and/or a profession. If the child failed, s/he went to the SM school to learn a trade.
Very few children from my little village school passed their 11 +. This was not because they were not academic, it was because there was no National Curriculum in those days, and teachers did not teach the children what they needed to know to get through the assessment.
If the children did pass the 11 + (my aunt was such a child), they could get some good qualifications, but they were unlikely to go to university for all sorts of socio-economic reasons. My aunt left school, worked in Boots and trained to be a pharmacist. Her sister, who failed her 11 +, left school and then trained to be a nurse. Both girls needed to work; both benefited from further education.
The SM that I attended was a place where you went if you failed the 11 +. The teachers were, on the whole, teachers who had a good social conscience, but who seemed very disaffected. Discipline was a problem. Perhaps the teachers wished they had the prestige (and pay) of the teachers at the grammar school.
The ideology behind comprehensive education was really admirable. It sought to provide high quality, openly accessible education for all. It was part of the welfare state which wanted high quality, openly accessible healthcare for all and high quality openly accessible housing and social care for all.
However, such equality was never achieved and this is partly because society itself is inherently unequal. My children, like the children of so many parents, had very little choice about the school they attended. There was one local 'comprehensive' school in our town, and unless they had a specific need for out of area education, that is where they went.