Very few Comprehensives are actually that, mainly because Distance tends to be the qualifying factor in admissions. Unless the school/LEA has been very careful to draw up a catchment area that includes a range of housing, what tends to happen is that the best school in every area is the one in the best area of the town, where only people living in houses worth ££££ can go.
My DD goes to a truly comprehensive school. Until last year there was a test to be considered for admission. Scores were banded and an equal selection of pupils randomly picked from each of 5 ability bands. Also, they take 75% of their intake from inner-city local postcodes and 25% from a wider postcode range. The ethnic diversity is huge, as well as the ability range.
The school was private many years ago, and still runs on the same lines. The uniform is horribly expensive (but people on lower incomes get help, plus there is a thriving secondhand market used by all).
With such a range of abilities their results should be average, but they are the best performing school locally. The difference is that you have to make an effort to get your child into the school and they demand a lot from the pupils. There are sanctions for misbehaviour.
Then you look at the newspapers, and threads on MN complaining that their child has been sent home for refusing to wear the right uniform, or having an unsuitable haircut, with the parents whining "what does it matter if he doesn't wear a tie/she wears makeup?". Parental attitude is the biggest issue in school behaviour, particularly in Y7 and Y8.