It would have to be fairly rigid on the catchment area criteria in order to weed out pupils from the other two services who wanted to enrol.
I believe what he was saying is that many from the grammar & public schools choose to go to the comprehensive because they actually have a much better time, and almost no one leaves the comprehensive because they'd be mad.
The Grammar School
Pupils spend all their time on field trips or acting as security on the school gates. Teachers don't like to leave the Staff Room except to go and ride horses.
Public Schools
Nearly all the classrooms were scrapped on the promise of getting two really big classrooms in ten years although one will probably go to a school in India.
They only have a few minibuses to play with, they had a few security trucks they borrowed off the Comprehensives but they had to hand them back.