Each county in Florida runs their own public schools so dates can vary. However so many counties were beginning their school year earlier and earlier that it got completely out of hand and the Florida legislature passed a law stating that schools cannot begin any earlier than 2 weeks prior to Labor Day weekend, the first Monday in September.
Thus, I do believe that most public schools in Florida will begin this year on Monday, August 20th. That would be for Kindergarten through high school Grade 12. Florida universities may be different but I'm aware that the University of Central Florida in Orlando is also beginning on August 20th.
Schools in the northern U.S. tend to begin school the day after Labor Day. So theme park attendance drops significantly when Florida schools begin and then really drops off after Labor Day weekend.
BobK/Orlando