[kiba]

Summer vacation is only detrimental to poor kids. In wealthier households, the children get sufficient mental stimulation so as to come back to school without fallen behind.

Maybe so, but wikipedia indicates that is a problem not just for poor kids, but also middle income kids as well.

The problem with changing school hours is that getting children to school needs to work around parent's work hours. This is less of a problem in other countries where young children can walk to school on their own or in groups, but would require massively changing American society to work.

For older students, this is not much of a problem, but smaller children does indeed provide significant challenge. However, given that adults are also continuously sleep deprived too, society will probably gain a lot from shifting their schedule.