err..you mean "greatly AFFECTS job prospects", surely? sorry to be pedantic but after all the thread is about literacy.
What are you actually asking? are you asking what factors are responsible for poor levels of literacy in certain socio-economic groups? because if you are it's a bit disingenuous to then go on to say schools and parents have no responsibility.
Your point that " it's no good expecting families to speak in a different way"- there is a direct, proven correlation between the way parents and their children speak.
Kids will copy their parents. But does it need adjusting by schools.? It is to some extent.. Children talk differently to their mates than they do to the teacher. The phone voice is commonplace. Whe I went on radio a lot. I talked a bit like an establishment wallah (Which I am not) The Q sticks with cut glass accent. Harry boy and Wills have modified. So has Camerooney and Blair baby. It can be attractive as with Frostrop. Bakewell tart , Gabby Logan is a bit too measured. (not born in Surrey) Old Muggeridge used to make people cringe; but he used the language brilliantly. (lst vol Chronicles of Wasted Time, best use of English ever written in my opinion.
WHATS TO BE DONE ABOUT ILLITERACY? Governemnts have asked that question for 50 years. Most prisoners are illiterate. Worth doing summat abaht that lads and lasses. Jewellry ,Jewellry....James saville was hard to place linguistically. Pigeon coal miner I suppose/
Lets have fun.....and a few practical solutions. (Send working class 7 year olds to public schools. See what happens)
I doubt it would be fun or useful. A large number of such children might serve to reduce all to their lower common denominator. Byt 7 its too late. Its starts much earlier than that. As the Jesuits once said " Give me a boy UNTIL he is 7 and I will show you the man" has more than a sprinkling of truth.