I was taught in a grammar school in the 1960s, and taught in secondary moderns in the 1970s. In the grammar school, the emphasis was on the potential university entrants. The least able pupils were ignored, usually leaving school at 16 with little to show for it. Very similar pupils in the secondary modern, however, benefited greatly in confidence and aspirations from being among the most able in the school, and often performed well in exams. I used to be saddened by parents who would try to get their marginal (in 11-plus terms) children into the grammar, when I knew that they would almost certainly do better in the secondary modern.
While I was teaching in a small secondary modern, I recall being asked by the director of education if it would be better if the school were a comprehensive. I replied that I didn't think so, because all that would mean would be the addition of one class per year of more able pupils, who would command much of the attention of the staff and therefore take the focus away from the other pupils. Of course, that may be less of a problem in a large comprehensive but they bring their own problems.
Any school system has its pros and cons. Whether or not secondary moderns did well by their pupils usually depended on the areas they served - still true of comprehensives today.
A G Williams
Quarry House, Start Lane
Whaley Bridge, High Peak