Prior research has demonstrated that summer learning rooted in family and community influences widens the achievement gap across social lines, while schooling offsets those family and community influences. In this article, we examine the long-term educational consequences of summer learning differences by family socioeconomic level. Using data from the Baltimore Beginning School Study youth panel, we decompose achievement scores at the start of high school into their developmental precursors, back to the time of school entry in 1st grade. We find that cumulative achievement gains over the first nine years of children's schooling mainly reflect school-year learning, whereas the high SES-low SES achievement gap at 9th grade mainly traces to differential summer learning over the elementary years. These early out-of-school summer learning differences, in turn, substantially account for achievement-related differences by family SES in high school track placements (college preparatory or not), high school noncompletion, and four-year college attendance. We discuss implications for understanding the bases of educational stratification, as well as educational policy and practice. (Contains 4 tables and 9 footnotes.)