It is usually the exposure to the neighborhood violence and family dysfunction. I have worked in low income schools for 16 years and also in some higher income ones. The teachers in the low income schools do tend to be less experienced, but they also have access to twice as many resources. Plus, their whole attitude is that they are willing to go the extra mile. Because kids come with so many obstacles, it tends to be the teachers that want to break the poverty cycle that stay at these schools. Still, we know that there is an IQ prediction based upon income, but on average, lower income parents have lower school smarts because they tend to have less education. On average, more parents have been in jail or on drugs, but that does not mean that these kids cannot get out of the cycle. There are gifted and talented programs everywhere. What they need to overcome are the social pressures from their neighborhoods. In some communities, kids that do well in school are considered to be rejecting their communities.
I do see kids make it, but the families in this case are very different. The parents are typically working lower paying jobs, but they make time to meet with the school and they make sure that homework is done. The children from these families that are able to resist neighborhood pressures can meet high expectations and graduate.
Generational poverty is such a huge issue, but it is primarily a social issue. To research poverty, it is nearly impossible to tease out the other social and family and biological aspects of it. When I started grad school, this was what I wanted to study, but it was too difficult to pull together a research project that a grad student could fund and get approved.