The first year of secondary, when they were waiting to set us for the second year, was pretty much a complete mess. Quite frankly I feel sorry for teachers who are trying to teach say french to kids some of whom remember what they did at primary - and some who can't remember mon, ma, mes, ton, ta, tes, son, sa, ses however hard they try (it was one of my friends - and I spent half an hour trying to help her remember them). Kids with different aspirations in the same class was enlightening - 'boring' kids like me, and ones who were already drinking and having sex behind the youth club aged 11. The kids who were bright did absolutely sod all or worked and got bullied (and those that did sod all to avoid the bullying got into such bad habits that they found it hard to throw them off).
Would I have been better at a grammar school - yes without a doubt. I left primary loving learning, with unusually high concentration levels, and so enthusiastic about secondary school. I left secondary having done well thanks to transferring to a private grammar for 6th form. But 5 years doing the minimum amount of work so that any success was seen as one up on the teachers left its mark (I did homework / coursework in the morning during registration, fell asleep in lessons from boredom, read magazines etc!). I have never been able to apply myself diligently again - the habits got ingrained.
And would my friend not have spent hours in frustrated tears, if not comparing herself to me and the others who found it easy - again probably yes.
My mother went to a girls grammar school in the 1950s - and from low achievement families with no background in university education, the opportunities and confidence those girls got was brilliant. I also know of people of that generation who were transferred over from the 'secondary moderns'.
If there were more grammar schools, then surely the tutoring would get less crazy, and the kids who were from disadvantaged backgrounds would get more of a look in. I can see the difference between my mother who was in a grammar school area, and her cousins who were not - and it is amazing. As I understand it the 50s were the time of greatest social mobility and aspirations - and maybe we should be thinking of that as a proportion - as not everyone can be, nor should they be pushed to be, academically successful.