Let's put it this way: If I lived in the 'hood with a crack whore for a mother and had no clue who my father was, and my grandmother was dead because my grandfather beat her to death (and he was on death row), do you think I might have different educational expectations (or a different emphasis placed on my academic achievement) than if I came from a middle class family with two parents (not necessarily together, but at least two parents who were both out of prison and drug-free) living in a suburb?
If mom's a crack ho, she's going to probably pay less attention to my grades than mom would if she were a middle class woman without a drug habit to feed. Even if I were equally bright, regardless of my socioeconomic standing, I'm pretty sure that I'd suffer if I lived in the hood.
(Unless, of course, there was extra credit offered for knowing how to build a meth lab. :P)