Splanky, could there be a socio-economic factor there?
My junior high’s school district was made up of middle class white neighborhoods and a couple poor neighborhoods primarily composed of black and hispanic families. As such, the school, while diverse, was composed of kids from different types of separate neighborhoods, the 40-50% white population being in the nice neighborhoods, and the 20-30% black population and 20-30% hispanic population being in the poor neighborhoods.
The vast majority of troublemakers, detention regulars, remedial students, violent kids/bullies, class skippers, etc., were black and hispanic. Of the few white kids who also acted in such a way, just about all of them were of the few white kids from the poor neighborhoods, and all of them hung out with the kids who lived near them. All of the black and hispanic friends I had in junior high were good students and most lived in the nicer neighborhoods.
While one could argue that it is a cultural thing concerning ethnicity, it could also be viewed as a cultural thing concerning socio-economic status, a poor culture, so to speak. In the case of my junior high, it seemed to me that it was more a matter of poor culture than ethnic culture, but because the categories of “poor” and “black/hispanic” overlapped so much, it could easily be seen as a matter of ethnic culture.
It appears that the study cited in the OP concerns a middle class area, and assuming that it is a middle class area unlike where I lived (which had separate middle class and poor sections), there could be an argument that ethnic culture is a larger factor than it may be. Still, given my experience (not that one person’s experience makes a rule), I can’t help but think that socio-economic situation and ethnicity are often closely linked, and that it’s hard to separate them when examining the cause of things such as poor performance in school. I guess we would have to wait for the book to come out to see how the study was made.
This thread does have some characteristics of an IMHO type thread, but I’m sure enough people will provide enough debatable posting material to keep it in GD 
shudders as he imagines being parsed, then nervously presses “Submit Reply”