The trick to IOA is to be at the gate before 9am when they open. When you enter the park, go clockwise on all the rides (except Popeye, it opens at 11am). Yeah, school is still in until June 14th. Field trips visit the park on a Thursday or Friday.
Here in Florida, it rains in the late afternoon around 4pm.
I checked on Yahoo, and Sunday and Monday look to be good. There is always a chance of afternoon showers.
You should have a great time. If it rains, then goto Spiderman and do it a few times. Or hop over to Universal and do some of the movie rides. The rain shouldn't effect them.