Most schools are out, I believe, early to mid June, so it's probably a moot point as a question of the less crowded time. It will probably be crowded both times you are thinking of. A couple of years ago we went the end of July and it was already very very hot, as I'm sure August is also. If you choose August, be sure to take refillable water bottles to the parks for each person in your family. Little battery operated fans are great to take too. (These are actually good ideas no matter when you go.) There are many indoor attractions too to escape the heat. It also helps to go to the parks early in the day, take a break in the afternoon to swim or just relax at your resort, and head back to a park around dinner time. The 10 of us in our family did this and were perfectly fine. What a nice graduation present!! Have a wonderful time!
