Realistically, if an area has very low performing schools, it takes a huge push to improve them all. Knowsley, just outside Liverpool, is the worst performing area in the country. There are now no sixth forms in the area and pupils have to travel to a neighbouring authority.
One of my sisters went to a grammar school in the area before it became comprehensive. That school is still the best performing school in the authority. I would imagine parents go to great lengths to get their children into the school, but those who don't get in are then left with schools where fewer than 37% achieve five A*-Cs at GCSE.
In reality, there are some seriously deprived areas in the authority and curriculum planners have a problem. They can provide a core of maths, English, science and functional skills type courses, which some would say are appropriate to the pupils, but others would say patronise the most able. Each school probably has too few pupils to make more academic courses viable, which is what's already happened with their A level choices. Alternatively, the schools could run academic courses for all, which wouldn't be appropriate for the majority.
When my sister was at school, she studied mainly academic O levels and went on to do A levels, but she would no longer be able to do those subjects. It's a real dilemma for schools in areas such as Knowsley. Ideally, the schools would receive extra funding for their brightest pupils, but that's not going to happen, because most schools spend extra money on their least able pupils and the brightest find themselves in bigger classes with less teacher attention.