Their tedious denial of the truth, boosted by the tedious fantasies of other ex-public-school boys, masks one fact- it is very difficult for people with one distinct upbringing to perceive of the reality of an upbringing of a completely different nature.
Before the days of Comprehensives (created with the largely forced closure of the Grammar, so that the parents of kids at the Comprehensives would include powerful middle-class types with high expectations for their children’s academic achievements), Mixed Secondary Moderns were largely seen as dumping grounds. Low achievers, disruptive kids, sexually promiscuous girls, etc. Not every school was like this, of course, but some were seen as little more than reformatories.