The real reason they start so early is to create a longer winter break, or what used to be known as a Christmas break. The reason they want a longer Christmas break is that half the student population is Mexican and they return to Mexico for a month or two December through January. The schools receive funds according to attendance. With a significant amount of the student population lazing in Mexico, the schools do not collect the funds they possibly could if the alter the schedule.
That’s not it. It is pushed by the teachers. They want to longer winter break.