I think that the big thing with good schools over bad ones isn't so much the individual grades your child can get, but the guidance of what to study and what to do next at college and university. I went to one of the worst schools in my county (I didn't know it at the time) but I was an 'A' and 'B' student at GCSE. What my school lacked was proper guidance as to what to do afterwards. My parents didn't really know, nobody had ever been to university in my family before, they just knew that I should try and do well at school. It was largely the same amongst my peers. Some people worked it out later in life, others (myself included!) never really worked it out.
Also, at my school, the main subjects (English/Maths/Science) were all done in 'sets' ('set 1' being the clever kids, 'set 2' the slightly less clever and so on). Is that not standard at all schools?
All I can say on Grammar Schools is that the evidence produced, by both those on the left and the right of politics, is that they produce worse educational outcomes for the kids who don't go in areas where they exist currently. I don't blame any parent for wanting to send their kids to the best school that they can, but it is surely the government's job to get the highest standard across the board, rather than higher for some and much worse than others (which is what the evidence on Grammar Schools suggests).