The problem is that people are prepared to demand more grammar schools, but ask them if they want more Secondary Moderns and I doubt if they would.
To use my personal experience, my children go to a large, very socially diverse primary school on the edge of two housing estates in an area of significant social deprivation. We have 50% more children on free school meals than the national average.
And who goes to grammar school? The children of professional, middle class affluent parents - that's who. And the children who really really need the step up go to the high school, which is good, but not as good as it should be because it doesn't have that top 25% - they are all off in their ivory tower learning Latin.