Wow - they go to school until late in PA! I've heard it's that way in NJ and NY also - almost to the end of June.
I've been during early June a few times, and it can get very hot. Just be prepared, go out as early in the morning as you can, then during the middle of the day, take a break in the air conditioning or swim.
I wouldn't worry about crowds too much - just plan ahead and you should be fine.
We have actually liked the end of August as well. It can be miserable hot, but it can be at home as well! The crowds are very low, as many schools in the midwest start in August, but we don't until September. You can get pop up showers, but they are usually short lived.