Nothing to do with "intelligence" as you well know. It's about you having a go at them for their grammar etc. rather than their views. It might not fit with your liberal fantasy of everyone having the same level of access to knowledge but the fact is that illiteracy is higher among the working class than among the other economic strata. This is unsurprising to anyone with a basic grasp on reality - if you have to spend much of your life just keeping your head above water you don't get the same chance to educate yourself as those who don't. Equally, lack of resources means that, even if you had the time, many educational channels are simply not available to you.
And so, when people see the likes of you making a big thing about bad grammar they think (as I do) "hang on a minute, their views might be repugnant, but in taking the piss out of them for their bad grammar, he's taking the piss out of me for mine" (or my friends, family, etc. for theirs). I have good mates who can hardly read or write. None of them are racist. Do you honestly think they'll just think, "it's ok, he's taking the piss out of illiteracy but only because they're racist - yes he's saying people with bad grammar are thick but he doesn't mean me" or might they think, "their views might be repugnant but he's not just taking the piss out of them - he's taking the piss out of me too."
It's not about abstract principle. It's about what's conducive to the advancement of your political ends and what's counter-productive. Your approach is the latter, and all your claims about being a working class teacher don't make the slightest bit of difference.
Nobody is saying that you're "making racists" but what you are doing is making sure people won't want to join anything that you, or those who use your arguments, are involved with. You're not making racists but you are putting people off antiracism. I know for a fact I'd never want to work with you because, and only because, of this. And if, unlike me, these people haven't met real antiracists they may well assume (perfectly reasonably) that we're all like you. So although they're against racism, they will not get involved because you make us all look like sneering liberal cunts.
And the way you assume people with low levels of literacy must be unintelligent is revealing. You completely ignore the differing levels of access to education between the working and middle classes (and that's ignoring conditions like dyslexia). But that's cos you're a liberal and so class doesn't matter - we're all just individuals and our educational attainment depends only on our intelligence and application, maaan.