When I first saw the correlation I began to think through the different reasons that might explain it. After stumbling past different graphs, I found some that provided me with an explanation. The families in the Southeastern US are more likely to be in a state of poverty. This causes the parents to push their children to go to school so that they do not follow in their footsteps (make a better life for them). Since many of the families in the Southeastern US are living in a single-parent household this causes different family issues; these issues arise and the students end up dropping out of high school or doing worse in high school for this reason. These unrealistic expectations from the parents cause the children to not be as educated as well. Or it could be that the education system is poor in quality down there.
The main problem I see in the Southeastern US is education. Neither parent typically has college experience, yet the parents tend to push their children to attend school. Whatever social pressures might exist down in the South, maybe farming or other natural societal causes, students end up not graduating on time. One possible cause is the fact that there are more single-parents down South. This could cause more stress for the students involved. But perhaps there being more heat and more sunshine affects this too. That is for another study, however.
Knowing what region of the US is the poorest, or the least educated, might not be that useful to some, but hopefully it serves to broaden your perspective. The more light we shed on this topic the easier it will be to address the issues.