From now until about mid September the parks are going to be packed. Don't worry about when school ends because people come from all over the world.
The only slow times are end of January - March and September - Mid November. All other times it's busy. If you are coming in the busy months, use the fast pass system.
Also, like fbi8904 said, don't expect to be cheap. In the park, fast food - burgers, fries - is $10 per person at the cheapest. If you want table service you're looking at $20 per person on the low end. Just don't be a tightwad. Even if it means postponing the trip until you save a little more money.
Still, it's one of the best vacations you can take. You'll see things you've never seen before and if you have kids, they will never forget it.
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