Some new research says no. Steven C. McMullen and Kathryn E. Rouse, from their piece ”The Impact of Year-Round Schooling on Academic Achievement: Evidence from Mandatory School Calendar Conversions,” in The American Economic Journal, report:
In 2007, 22 Wake County, North Carolina traditional calendar schools were switched to year-round calendars, spreading the 180 instructional days evenly across the year. This paper presents a human capital model to illustrate the conditions under which these calendars might affect achievement. We then exploit the natural experiment to evaluate the impact of year-round schooling on student achievement using a multi-level fixed effects model. Results suggest that year-round schooling has essentially no impact on academic achievement of the average student. Moreover, when the data are broken out by race, we find no evidence that any racial subgroup benefits from year-round schooling.
There is an ungated version here.