We go every few years in late august, and were just there august 23-26. The crowds are pretty good. The later in august the better. Many many schools are back early august, but as a Bostonian, we don't go back until September.
As for the heat, it doesn't bother me. 90+ and humid is much more enjoyable in disney than in boston. Do be warned that sudden thunderstorms can put a damper on water park days.
Have fun.
Edit: I should add that since you are used to October crowds, we find them quite comparable. Maybe a little lighter in august than October.