If you look at a school like Watford Grammar school for Girls despite being called a Grammar school. In the defination of type of school, in the Dept of Education Performance Tables is called a Comprehensive.
Watford grammar School for Girls only takes 2% low attainers 17% middle attainers and 81% high attainers it has academic selection and yet is designated as a Comprehensive. Maybe the reason it takes 2% low attainers is for legal reasons.
On the basis that Watford Grammar school for Girls is not really a Comprehensive school, what is a realistic breakdown in a affluent area in terms of academic abilty in attainment.