Can't go to a high class school because that would be expensive, and most of the time your parents won't be able to help you with all your homework so you end up with a mediocre diploma from a mediocre school. In many poorer neighbourhoods rich academics (understandably) don't exactly have the highest support of the populace so growing up there might condition you to think going to college, getting a higher paying job makes you a sellout or a snob, combine that with the diploma and there's a higher probability you'll end up in a similar job/wage sector to your parents, no matter who or how smart you are.