A couple of things I should add, perhaps. It's not that secondary moderns were/are "that bad". They were just never designed to be academic, but to teach practical skills (along with basic numeracy and literacy), that would fit people for manual and low-grade clerical jobs. They simply weren't geared up to get people ready to go to university. (It's because I went to an ex-secondary modern, for example, that I was never taught Latin, much to my regret.)

Secondly - although perhaps this was clear enough - in common parlance no one talks about the tripartite system, even though that's its proper name. It's always known as the "grammar school system".