Researchers have disagreed about the extent to which differences in achievement based on socioeconomic status (SES) accumulate during the school year as compared to the summer, and the literature has not fully assessed the contributions of social contexts--in the form of both school and neighborhood poverty concentration and racial and ethnic composition--to seasonal disparities in learning. We addressed these shortcomings in the literature by examining how family SES and school and neighborhood contexts explained differences in children's achievement growth during the kindergarten and first-grade years and the summer season between them. Data on student achievement, family background, and the school context came from the Early Childhood Longitudinal Study-Kindergarten Cohort (ECLS-K). We linked information regarding neighborhood social context by merging data from Census 2000 with the ECLS-K sample using students' home zip codes. We partitioned variation in reading and mathematics outcomes between the individual and organizational levels of analysis by using three-level growth models. Our findings provide strong evidence that differences in family SES were associated with reading and math achievement gaps during the school year, in both kindergarten and first grade for reading and in kindergarten for math. The totality of school-season SES disparities was larger than that for summer season disparities. Neighborhood social contexts influenced reading and math achievement outcomes at school entry and during the summer season, and school social contexts were more salient for reading than for math achievement. The impact of SES-based inequalities during the school year raises important questions regarding previous research on seasonal learning outcomes and suggests that schools may not necessarily serve as the "great equalizer." Moreover, we found that school and neighborhood social contexts exacerbated family-based learning inequalities in ways that resulted in a double disadvantage for many students from low-SES families and a double advantage for many students from high-SES families. (Contains 6 tables and 24 footnotes.)