My experience in the golden age of grammar schools in the 1950s was that the selective intake in general got a poor deal. A third of the intake was allocated to a general stream after two years and expected to have modest achievement. Even the elite science and arts streams were expected to have modest achievers, and the concept of some boys not being entered for any O-levels even in these streams was well-established.
I have to confess that, despite this ethos, I achieved some success for myself, and for the school's record, in retrospect, I probably received more general education and perception from the rag bag of scholars and eccentrics that then formed a solid cohort of the staff of an inner London grammar school. Others, including John Major at a similar school, had to seek success in life after leaving with minimal qualifications for five years' study. Surely this is not the type of school we wish to re-create?