Last year I went the last week of Aug and the parks were empty. I don't think that is because just FL goes back early but lots of the country goes back before Labor Day. In fact, I would say nearly 1/3 of Massachusetts now goes back before Labor Day. I don't know for sure how many do but I have lots of friends/family that go back before Labor Day. Our town starts after Labor Day. But even those that aren't back in school are too busy getting ready for the start of school to go to Disney. I am planning another trip for the last week of August this year. I'm more worried about a hurricane than crowds.
Massachusetts requires 180 days of school (do other state?) and also has a law that says kids can not go to school in July. We had so many snow days last year they had to cancel a teacher conference day, Good Friday and went to school til June 30th. I don't think we had any snow days this year.