Here in Maryland by law students have to be in school for a specific number of days each year. So theoretically, if school starts early, they can get out earlier in the summer.
However, if days are missed because of the weather, then they have to stay in school extra days to make them up. For example, the 2009-2010 school year we had a quadruple whammy. "Snowmageddon" caused massive school shutdowns all over the state. That led to possibly having many extra days at the end of the year to make those up. The state legislature had to approve the kids not getting all of their make up days, otherwise they would have been in school until early July.