I think that's a different argument and far more about class, and also one that positive discrimination and hideous 'quotas' have changed the parameters of hugely.
With the exception of all women shortlists for MPs it would be difficult to have quotas/positive discrimination, as the legislation is symmetrical and therefore any such action is unlawful. It doesn't benefit women or ethnic minorities any more than white people, except that there tend to be more instances of discrimination against them. I was taught that the first ever case under one of the sex discrimination Acts was brought by a man, who won. Since the 2010 Equality Act it's apparently been possible to choose a minority/underrepresented candidate on that basis if and only if two candidates are of equal merit, but that's a long way from positive discrimination/quotas - it just recognises that altering cultural biases take new role models and a widened pool of employees/managers in more senior roles. All the law change did did was make that practice lawful if it was one an employer wanted to use; it didn't mandate the practice. The idea that we are "awash with PC nonsense and positive discrimination" is a myth - a very convenient one, for certain groups.
I agree abuse can affect all classes, and that middle class kids are in some ways less protected because it's a lot easier, seemingly, for the social services to overlook such abuse if the parents are middle class. But it's compounding the harm when the victim is also poor, because they miss out educationally, possibly nutritionally and also very possibly culturally. Enriched lives cost money, or at least the sort of cultural capital that is a legacy of parents/grandparents with money. Intersectionality, basically. There will always be exceptions to any general rule (my DH is one such grammar-school educated example - immigrants on one side of the family, the Doncaster area mines on the other), but that doesn't invalidate the principle as being the norm, either. You also talk about your terrible school, which I accept, but then mention your parents were well off. Presumably, they were literate themselves? That's cultural capital, and pretty valuable to a child, whatever their clear failings in other areas.
LiberalPedant, I think it's snobbery. A socially acceptable form, because it's ostensibly about smarts and effort and not social class background, despite literacy being so clearly linked as a rule of thumb. Critical thinking skills are a whole 'nother story in my experience, but simple illiteracy is rare amongst neurotypical middle class kids. And it interests me that there's a divide between acronyms (fine) and txt speech (not fine). I mean, people here would be appalled by someone posting using phone txt abbreviations, while blithely referring to DTD, DD, LTB, YANBU themselves. Truncated forms of language are truncated forms of language, surely? Someone told me recently that txt speech became socially unacceptable amongst educated young people shortly after predictive texting arrived, but wasn't yet anything like universal. In effect, txt speak for that brief window of a generation indicated an inability to afford a fancy handset. I've no idea whether that's based on research, but it's an interesting point, and similarly, acronyms are frequently used in a professional context, which perhaps promotes social acceptibility. People abhor "hun" here, while happily referring to their "darling daughter" as "DD". Not much to choose between them, surely?
It's also interesting: I frequently see very literate people wail, "My maths is terrible! I count on my fingers!" and nobody thinks anything of it. I can't imagine those same people saying, "My English is terrible! I can barely read!" There isn't anything like the same judgement attached to numeracy. I'm not sure why.