Application of the Durham formula et al?

[Tolstoy]

I understand how this can work for Comprehensives but how does it work for Upper schools/ secondary moderns in fully selective areas? Do they still apply the same criteria even though these schools would be expected to have a lower percentage achieve high grades regardless of teaching standards due to having a large chunk of their high achievers removed?

There are obviously some high achievers who attend these schools for various reasons but who may not be given the same opportunities for success as those who attend a Grammar in the area.