Longer school years are nice, but I think longer school days might be better, especially for underprivileged students in disadvantaged environments. For kids growing up in these areas, one of the big problems is just the absence of any meaningful order.
I think this is exactly right. If, like me, you see poor school performance as largely a function of poverty and the various socio-economic issues associated with poverty then keeping kids in an environment that fosters learning for a longer period of time is critical.
It's not a quick and easy fix though because it costs money to have more teachers, longer hours, more hours with the lights on, etc.