most of the florida schools went back this year on August 11....that's pretty typical for them...and many southern states go back in mid-august, so you'll be safe with the second half of August....we were in WDW the week of August 16th to August 24th and it seemed practically empty (especially after our visit just the month before in July, when it was psychotically crowded)...
as for the person in Michigan.....all the schools in the district where my family lives go back after labor day and always have (at least for the past 50 years, which is always as far as i'm concerned)....i don't know where you live, but i'm in suburban detroit, where the schools will go back this week (after labor day).....
which brings to mind, you can't generalize for US schools and you can't say anything definitive by state....each school district makes its own rules...