I was thinking something similiar. In the days before airconditioning, holding classes during the heat of summer would be problematic. I can easily see teachers complaining that not only was it hard on their own selves, but students simply couldn't be made to learn in that sort of environment. Most of them would be falling asleep every few minutes.
And not just 'back then' cuz' that's exactly what happened in more recent times when the 'genius' admins, sitting in their robustly a/c'd offices, decided they had to "keep up w/ the Joneses" & start classes in the heat of the summer. They found out that the schools hadn't been designed to be full of hot, even over-heated bodies during that time of year. I once talked to a teacher from SDakota who told me about when the start of school was moved to mid-AUG over there. The schools had not been built w/ a/c (not needed in SEP, according to her) while, in AUG, the temp would often go over 90F, with predictable results that were only surprising to the Ivory-Tower-Eggheads that cooked up the scheme!