It actually made no difference to outcomes for girls in my area.
Both schools offered O and A level. The secondary in fact offered 3 routes. Academic, from where girls went on to university, or teaching or entered a profession requiring As at the time. - pre-nursing which worked with a local hospital and girls guaranteed a nursing training course at the end of their As and what was termed a commercial course where girls were taught shorthand and typing to a high standard, alongside their o and a level courses that concentrated on the commercial world. Girls following this route entered the commercial world with skills and knowledge of how the commercial world worked from banking to the stock exchange, from export to retail etc. They were highly valued and expected good salaries. There was of course the less academic route where girls were taught extremely practical skills, like childcare, domestic science, art of various kinds like pottery, basket making etc.. Don’t forget we are talking the50s here, so no traditional boys stuff taught.
The grammar was more traditional and less imaginative in its courses. It was academia or simply leaving with Os or As to a job. I think the girls were short changed.
So really you might argue that for a career in the late 50s you were much better off going to the secondary modern.
That in fact the outcomes were better.
I failed my 11 plus but went on to university nevertheless.
The headmistress of our school was outstanding with the imagination and flair that made 100% difference to girls lives.
That is why, in my view grammar schools offered too narrow an outcome with limited choice, certainly in our area, and knowing friends from both schools it appears that the ex-secondary schools girls had both more interesting and varied careers,