Part of the function of a school is to free parents time so they are able to go to work. Even high school essentially is in large parts teenage daycare. This function gets diminished when they start later in the day - meaning economically, you diminish the potential output of your population.

Schools that are up to date on this and follow advice exist, but are expensive and rare - which also attract parents who do not need to sit at a desk or stand in front of a CNC machine at 8:30am. Gordonstoun/Schloss Salem comes to mind.