There is no such thing as a 'comprehensive' school.
Every school is a product primarily of the parents of children at the school, with the exception of special schools where children may have genetic disabilities and so on.
Private schools will beat state schools because they have more resources and won't have to deal with parents who aren't interested in education.
Selective schools, such as Catholic schools, beat those without such policies because again they successfully filter out bad parents.
Higher achieving state schools will be targeted by good parents, and again this makes those schools better.
Finally you've got schools like the one my DS was allocated to (he's going private instead), special measures, hopeless, where most good parents will avoid it, leaving the school with a very high proportion of bad parents.
Talk of abolishing private schools, grammar schools is therefore absurd, because until we invent personal teleportation devices, schools' makeup are determined by geography - somewhere in a leafy part of North Yorkshire will be dominated by middle class parents, somewhere in a rough part of Stoke won't be.
So it's absurd to say all children will be educated together. They cannot be.