"Best" is a combination of... - minimizing cost to the district (number of buses, etc)

- high school starting later in the morning (adolescents need more sleep)

- ensuring adequate child-care options are available for little kids. Early start means more PM care, late start means both AM and PM care, etc.

Then you get compounding factors... - high school athletics. Parents don't want high school to start too late, because then there is no daylight for football practice (yes, this is an actual argument I've heard... parents would prefer their kids play sports than maximize the kids education). - race and income issues - either correcting existing disparities or avoiding creating them.