Most schools in the midwest US actually build a school calendar that is 7-8 days (or so) longer than is actually necessary. That way if they have to take snow days, they're not going until July; alternatively, if they don't take snow days, everyone gets excited to be let out early. . . when it's really the legal time to be out of school.
I've never heard of a school actually letting out early though. Last year my son had to go an extra week though because they used more than the planed days off.
