Long ago, most classrooms in Texas weren't air-conditioned. Maybe the principal's office or some windowless area, but not all of the school. School started around Labor Day and ended around Memorial Day. We sweltered for a small amount of the year.
With the advent of air-conditioning and the continued pandering to kids ("let's let the Christmas holiday be between terms so they won't have to study") gradually the start of school slid back into the summer. Then people noticed a loss of vacation revenue at beaches and amusement parks. And the cost of cooling in the dog days of summer. Now, in Texas, there has actually been legislation to push the school openings back toward Labor Day.
One of our readers notes that there is a fallout at the other end of the school year. As part of the continued pandering to kids, elaborate graduations in big (air-conditioned and comfortable) venues are booked years in advance. Only they are now scheduled too early! In my day we graduated in the football stadium.
When you move the date one way for some 'good' reason it has economic consequences. Correct that and, of course, other unintended consequences raise their heads. In Colorado where my great nephew goes to school, kids are using the schools year round. And, of course, you don't need as much air conditioning.