The more important question is, how difficult is upward social mobility in 2018?
In fact, some of them are referred to in the study itself as an example of the debate they are addressing - "Advocates of the community-focused approach argue that teachers and school administrators are dealing with issues that originate outside the classroom, citing research that shows racial and socioeconomic achievement gaps are present before children enter school (Fryer and Levitt 2004; 2006) and that one-third to one-half of the gap can be explained by family-environment indicators (Meredith Phillips, James Crouse, and John Ralph 1998; Fryer and Levitt 2004). In this scenario, combating poverty and having more constructive out-of-school time may lead to better and more-focused instruction in school. Indeed, James Coleman et al. (1966), in their famous report on equality of educational opportunity, argue that schools alone cannot treat the problem of chronic underachievement in urban schools. "
Sure, but how do you motivate the students? Does that come automatically with a high quality school or is there more to it? How big a part of a achievement is the motivation and how big part is the school? If you take a failing kid from a poor school and 'force' him to go to a good school is that enough to get the desired improvements? Could you achieve similar effect by just focusing more on the motivation side?
While I do have lots of hunches and guesses, I don't see any obvioulsy self-evident answers to any of the questions.