"Lasting Consequences of the Summer Learning Gap" by Alexander, Entwisle, and Olson examines the long-term effect of differences in summer learning, which tend to be associated with family socioeconomic level. The researchers found that achievement during the first nine years of school is related primarily to school-year learning; however, the achievement gap between high-SES and low-SES students at 9th grade is more closely associated with differences in out-of-school summer learning during the elementary school years.
Alexander, K.L., Entwisle, D.R., & Olson, L.S. (2007). Lasting consequences of the summer learning gap. American Sociological Review, 72. Retrieved from http://brettberk.com/wp-content/uploads/2009/07/april07asrfeature.pdf