Adding to previous post-
Both primariy schools DD went to fed the same secondary school. This meant I've had two speech days in which to idly read the lists of exam results and, of course, I was interested to see how the pupils from the two primary schools compared. This is not scientific but I am an ex-teacher. I have known many of these children from pre-school age and had been a parent-helper in the easy-going school.
Both are rural village primary schools with an affluent catchment. The majority of parents are professional, many of whom are highly educated themselves. The secondary school has an excellent academic record.
As far as GCSE results were concerned, both groups did pretty well with roughly the same number getting 5 or more C grades and above, and roughly the same number getting 7 or more. However, the pupils from the 'easy going' primary who were struggling back then tended not to have done well at GCSE whereas the less able pupils from the 'hard working' primary tended to do comparatively well at GCSE.
Something that was glaringly obvious was that a disproportionate number of pupils from the easy going school who had done best at GCSE had joined that primary school from elsewhere. They had already learned to read before they came to that school and it had been obvious in Y4 and Y5 that they were streaking ahead of many of the pupils taught there from reception. I think it is true to say that the worst results were from pupils who had been at the easy going school from Reception to Y6, with one exception - see below.
More recently, I was fascinated to see the A level results for that year group. Four students had done spectacularly well - 5 or more A grades. Two of them were students who had gone to other primaries. Of the other two, one was my DD and the other was a girl she knew from the easy going primary.
I know her mother was as concerned as I was that the lessons at primary school were not stretching enough and had set about ensuring she had plenty of home stimulus. The difference was that this girl was older in the year group than my DD and the school always split classes by age, not ability, so my DD was frequently left to work with the year below whereas the other girl frequently worked with the year above. Our only recourse was to move DD to the other school. (I have often thought that if they could have been allowed to work together a friendship might have been formed which would have meant my DD was less isolated in that school.)
The rest of the A level results showed that a greater number of the pupils from the easy going school had left after GCSE, presumably to go to the 'tech'. Of the ones who had stayed in that sixth form, the results were pretty well comparable. Though I would say that those who did best from either school had parents who would be 'hands on' throughout.