I have always thought that it's all about the parents. Stupid parents tend to congregate in poor areas and have stupid kids who generally all go to the same school. And vice versa for smart parents.
There's really not much that a teacher can do with a classroom full of stupid and disruptive kids, and there's really not much a teacher has to do with a classroom full of smart and attentive kids.
LOL..well, I wouldn't say that! All kids CAN learn. But, the home life does make an impact. That's why all these schools willing to take kids from low poverty areas want parents to sign "contracts" that the parents will do this, that and the other thing. These school know without parental involvement, it's very tough.