I'd be curious to hear about any contrary evidence, though.

Perhaps another reason peer effects don't show up is that situations consisting of one kid of upper class background completely surrounded by lower class kids and having no other options but them as a peer group are relatively rare. In most cases there are a number of other middle classish kids in the same boat to form a peer group with.

I base this conclusion on two pieces of evidence, the first is anecdotal, my own school background. My school had a variety of kids that included a large amount of lower class kids from a nearby trailer park and a large amount of respectable kids. For the most part nothing the trailer park kids did rubbed off on me or any of the other respectable kids because we rarely socialized with them, we naturally tended to interact with the kids we had something in common with (although most of the trailer park kids were friendly enough in class, I can't really say that most of them were unpleasant to be around). The only long-term impact they had on me was to help me realize that the underclass are usually trying to be nice people, even if they fail at it a lot.

The second piece consist of articles (mostly by Thomas Sowell) I've read about various immigrant communities in poor neighborhoods and how kids from groups with middle-class values (i.e. Chinese, Jewish immigrants) tended to cluster together and interact with each other and not the poorer kids around them. So it seems plausible to me that the ability to form small clusters of like-minded peers might mitigate peer effects.

Also, I second jsalvatier's points about some of your comments having a "political" feel. In particular it seems like you have a tendency to work in angry-seeming statements about how awful and unpleasant poor people are that can be rather off-putting, to say the least.