It seems like most kids in the southern part of the country go back to school in August.  I think most of the schools in the northeast don't go back until after Labor Day.  
We went the last week in August.  I was a little apprehensive during the summer months as I used to live in Clearwater and it always seemed to be a "madhouse" over there.  I was pleasantly surprised and didn't see the huge crowds I was expecting, even with the FTP that was offered this past year.
MK was the most crowded place, but even that park wasn't that bad.  We had a blast and I hope to go back again in the next few years.
Based on that experience at that time, I would say plan on going at the end of August - the value season also starts then (I think.)  It was hot, but it's mostly the same temperature there as it is at our home.  The bus service was great.  The weather cooperated - there were always the afternoon storms (one turned into an evening storm) but that's always to be expected down there.
Have a good time!!