Our four children all attended same-sex grammar schools in our county, and benefitted from university educations, one at Oxford. Two have Masters degrees.
My husband and I are not privileged, elitist, or academic. We are what used to be known as 'working class'.
My father had little education. My mother went into 'service' aged 13. I attended a girls' secondary modern school during the 60s until I was 16. There was no money for further education. Neither my husband nor I had university educations.
Our children went to the local village state school. They took the 11+ back in the day before preparation, coaching and private tuition were encouraged, if not expected. In fact, the day of the test was kept secret, so that children would not feel anxious beforehand. There were a few test papers completed at school, to give pupils a rough idea of what to expect, but that was all. It was never seen to be the 'big deal' that it appears to be nowadays. It was viewed as a measure of the child's intelligence, and whether or not a grammar school would suit their level of intelligence .
This is where, I believe, the grammar system is no longer a level playing field. Those who have the financial means can ensure that their children get a head start by paying for private tuition. The actual method for passing the test can certainly be taught; however, this does not help a child who may then struggle with the expectations that a grammar school may impose.
In essence, I do feel that areas that still support the grammar system help to elevate the standard of local schools generally; primary, middle and secondary. Our children's state primary and middle schools were/are excellent, possibly, in part, due to an ethic of encouragement and the will to see all pupils achieve their best.