I took this info and cut and pasted it from the Florida Department of Education site. The start date listed here is 2012 but the law has not changed. Following this law would move the earliest start in 2013 to 8/19/13. If it helps any I have students in both Manatee and Sarasota schools both start 8/19/13. Sarasota is one of the few districts that is exempt from this law but is still following it for the 2013 start date.
In 2006, the Florida Legislature adopted a change to Section 1001.42(4)(f), Florida Statues, regarding the opening of the school year. According to the statute, beginning with the 2007-2008 school year, the opening date for schools in the district may not be earlier than 14 days before Labor Day each year. However, according to Section 1003.621, FS, districts that are currently designated by the State Board of Education as Academically High-Performing School Districts have the authority to waive Section 1001.42(4)(f), FS. This year, August 20 is 14 days before Labor Day, and 42 of the 67 regular school districts in Florida will begin classes on that date. Nineteen districts Brevard, Calhoun, Charlotte, Citrus, Clay, Flagler, Gilchrist, Gulf, Lee, Leon, Martin, Nassau, Palm Beach, St. Johns, Sarasota, Seminole, Sumter, Wakulla, and Walton are exempted from the statute because of their status as Academically High Performing districts. Fourteen of these districts will start between August 7 and 16. Four districts Hillsborough, Putnam, Union, and Bradford will begin between August 21 and 29. The remaining 7 districts have not yet reported their 2012-13 dates.
This near uniformity in start dates stands in stark contrast to school years prior to 2007-08, in which the average difference between the earliest and latest start dates was 23 days. The longest stretch between the earliest and latest start dates was 34 days in 1997-98. For 2012-13, Brevard County has the earliest closing date (May 22), and 7 counties share the latest closing date (June 7). Sixteen counties close on June 6, making it the most common closing date.
I also agree with the two other posters even late August will see tons of famlies at WDW because there are tons of places where school does not resume until September. It's doable but I highly suggest early starts and good use of Fastpass.