I'll parrot what others are saying as true.
In mid-August, many-but not all-schools are back in session. The major holdout is schools from the Northeast. The parks won't be deserted, but considerably less-crowded than the rest of the summer, and almost-certainly nothing like you would experience during a holiday like Easter or Christmas...
Now, having said that, the first Monday in September is a national holiday-Labor Day. It's usually the "last hurrah" of the summer. Usually, all those still-closed schools in the Northeast open one or two days after Labor Day. Many families take advantage of the three day weekend and go to WDW.
Here's my guess:
You're arriving approximately on the 18th. It's a weekend, so there will be lots of locals there as well as vacationers. Plan on going to Epcot; it tends to be the least crowded park on weekends (except for the big guns like Soarin' Test Track & Mission Space, but even then that doesn't often mean large crowds everywhere else).
When Monday rolls around, and until Thursday or Friday, the crowds will seem really really light. Indulge as best as you can.
The weekend rears its ugly head, go back to Epcot, or just hang out poolside and re-energize.
The following weekdays will be busier than the week before, and will get busier the closer you get to the weekend, as more people arrive for a 3, 4, 5 or even 6 day Labor Day Weekend. The crowds should still be tolerable, but not as sparse as your first week.
The major wrench in those plans could be the fact that it'll be Hurricane Season. It could keep people from coming in, so fewer crowds. But they might have to close the parks, too. If you're getting a villa, be on the safe side and at least stock up on some foods and movies in case you're stuck inside one day weathering the weather. And, again, plan on breaks from the weather, which will probably be absolutely brutal.
