One of you said it doesn't matter if it is hot, because the school day is spent 90% in school. THAT's WORSE! A lot of schools do NOT have any air conditioning. There have been times where the teachers would have us sit on the floor, with fans on and lights off. It is miserable, and hard to learn. So, I would definitely have enjoyed starting school after Labor Day. But, I never minded starting in mid-late August, because as mentioned, it gets out out even earlier.
Either way, you are going to deal with heat, because if you go in Early September, you'll have to stay into June.