Huh, it is pretty much settled that kids from "good homes" have better school results then the ones from "bad homes". The kids themselves are under less stres, they eat better, they sleep more, they have someone who cares about their school results. The parents have more resources (time, ability, inclination) to intervene when things go badly regardless of what school does.

The Gates Foundation documented that underperforming students tend to actually gain on the average throughout the school year.

The problem is that those same students lose terribly over the summer break. Students from better socioeconomic strata gain over the summer break while those of lower socioeconomic strata do not. Thus, when the school year begins again, any gains made over the school year have been more than cancelled out over the summer.

This is one of the big arguments for year-round school.

The kids from good homes perform better.