I have a number of US friends. Their kids start getting out after their May long weekend (states like Oklahoma and Iowa seem to be early) and then it varies by state. The May long weekend for them is usually the weekend before our May long weekend.
It's hard to guess when WDW will be quiet. We've often gone in April and done well but I've heard that April this past year was busy. We like to go the week after the week Easter Monday falls in. That week we run into grad nights but that only means MK closes early one night for their party. We haven't found the teens to make the parks crowded or cause any trouble. And the weather that week has always been fabulous.
Check the WDW reservations page. They show clearly what weeks are peak or low. I feel like, since they know of every conceivable event going on in the parks or environs, their calendar is probably the most accurate.
