The difference starts well before students are enrolled.
Prior to comprehensive schools, we had two main types of teenager education, the secondary modern and the Grammar school.
Before that, we also had the indentured apprentice - which was a trade skills apprenticeship starting at age 14, however that really was a long while ago.
There was a selection process to differentiate between the secondary modern and the grammar school.
This was known as the 11 plus exam, if you passed then you went to Grammar school. In later years a second chance was offered called the 13 plus exam.
These were not exams in the current sense of the word, it was not just case of attaining a certain mark to pass, this was a competition, so the top 5% would go to the very highest and most academically desirable Grammar schools, and the next tranche of maybe 25% went to the other Grammar schools - all schools had some sort of perception by the public.
If you went to those Grammar schools you would be chalked up for higher education, by being streamed into the old O-Level and A-Levels, that top tranche of Grammar schools were pretty much the golden path to the highest universities, it was very much an expectation that you would be in the running for the professional classes. The other Grammar schools would offer the same qualifications, and a path to university, but they didn’t quite have the same reputation - especially if you wanted to try for Oxbridge. Grammar schools were very much designed for academic education.
Secondary moderns varied in quality immensely, far too many were just dumping grounds for parents to leave their children whilst they went out to work. Those schools would generally be found in the catchment areas of the sink hole council estates and had absolutely terrible reputations. Parents often had little choice but to send their children there because they were unfortunate enough to live in the catchment areas - at that time parents were not offered a choice in where they could send their children to school, and many able students lost out on life chances by being made to attend these educational waste pits.
Not all secondary moderns were this bad, indeed many of the inner city ones had very good reputations and many notable alumni. You could take on the academic qualifications in many secondary modern schools but this was reserved only for the highest performing minority, and the remainder would be enrolled for the lower standard CSE qualifications.
The requirement that students gain qualifications was not anything like a significant priority in secondary modern schools, and the majority left those schools with not one academic qualification whatsoever.
During all this time, schools were almost exclusively single sex.
Those who went to grammar and then on to university tended to be from the children of the professional classes, with a smattering of others from the rest of the workforce, and their university fees were covered by public funding.
It was really pretty obvious by the late 1960’ that this was grossly unfair, that grammar school pupils had more resources ploughed into their education, and that one exam at 11 years old could set the entire life chances for an individual.
Its was also obvious that we were really only educating a small part of our population academically, and that the rest of the world was making far better use of their education systems to produce a workforce that was capable of responding to change.
The comprehensive was aimed at getting rid of the 11+ competition, and instead all students were to be streamed according to their abilities, rather than a one off test. The idea was that able students could be moved into higher streams at any time no matter what their background, and at the same time there was a great increase in the use of mixed gender schooling too.
The idea had been that the good students would drag along the poorer performing ones upwards, but of course the exact opposite happened in many cases. Sink hole schools remained sink hole schools, which they still do right up to this day maybe 40 years later.
There are those who yearn for the grammar school as a means of keeping the best performers in an environment and academic culture of performance, when the reality is that Grammar schools were always a form of social apartheid that helped ensure that those from the lower order remained there, no matter what their natural abilities.