This I will defend. Someone can correct me if I'm wrong, but I think dismissals for heat happen much less frequently now that more buildings have AC. But having grown up in schools without it (and I'm not even that old) it was miserable and it reeked. You aren't going to learn anything in that environment.
As for cold, last winter it got so cold a couple days that buses wouldn't start. That alone pretty much seals the deal.
None of the schools I attended had AC. Two a day football practices started the same week of school so we would have a 2 hour practice in that AM and then go sit in a hot school all day. It was absolutely miserable and the afternoon practice would just blow.