Schools in the south tend to be out in May, schools in the north generally aren't done til mid June. Year round schools still only make up a small percentage of all schools so that shouldn't have a big effect. Here in the DC metro area schools aren't done til close to the 20th of June. My 2DS are in college so I'm not 100% of the schedule anymore, but around the 18-20th was always the most common timeframe, depending on how many snowdays they used. FL students only make up a small percentage of visitors. The summer is just more crowded, and that starts as schools start letting out in May across the country. Also remember, foreign students have different schedules than we do. From what I've heard the parks are just getting busier year round.