MOnica, I did mention in an earlier post that before 1944 approximately a third of places in fee-paying schools were actually free, so I do know that all parents paid fees. However, the situation wasn't uniform across the country. If a child was bright, he or she had might have had the opportunity to sit a scholarship exam. If not, the parents could pay for the child to go to a private school. If the child wasn't bright enough or there weren't any nearby schools offering scholarships or the parents couldn't/didn't want to pay, the default secondary school was elementary education, which finished at 13 (later 14) and only offered a basic education. Girls' education had far fewer opportunities.
After 1944, the elementary schools became secondary moderns and the others jostled for a role for a while. Some of the more academic private schools became completely independent (with some still offering scholarships) and others became direct grant, with 25% of places' being free. Some of the less academic schools remained independent and survived because some parents didn't want their children to go to secondary moderns, if they weren't bright enough to pass the new 11+. Many of those schools have now disappeared, as the new comprehensives were introduced and improved. The Catholic Church paid for and maintained a number of schools, presumably in the hope of offering Catholic children a religious education and possibly producing some future priests. It's one of those kind of schools which John McDonnell attended.
The original seven HMC public schools always were something different and in a totally different league.
I'm well aware that it has always been possible for "working classes" to rise up the social scale through education, but it wasn't universal. The late nineteenth century onwards saw a number of people rise up through the social ranks and there was some mingling with the established aristocracy. It was a fascinating time. Like yours, MOnica my parents' families did just that. One of my gt gt grandparents was a builder who made oodles of money and decided to spend the money on education for his nine children, including the girls, one of whom was one of the first women to achieve a London University degree. My grandfather went to one of the original public schools and became the head of a civil service department by passing exams.
The above was how meritocracy worked to challenge the old aristocracy. However, it wasn't ever universal and has always meant that the ones who didn't rise to the top were neglected.
Sorry for the length of that post. The point is that the school which McDonnell attended wasn't "posh" and would have been much more commonplace than it is today. Religion was more important than it is today, so for many poorish families, supporting a child to become a church minister was seen as a way out of poverty. That's why the Catholic Church paid for places at its own schools and why parents were keen for their bright children to take them up.