As social and economic stratification between black and white Americans persists at the dawn of the twenty-first century, disparities in educational outcomes remain an especially formidable barrier. Recent research on the black/white achievement gap points to a perplexing pattern in this regard. Schools appear to exacerbate black/white disparities in learning while simultaneously slowing the growth of social class gaps. How might this occur? Using 1st grade data from the Early Childhood Longitudinal Study-Kindergarten Cohort (ECLS-K), I test--and find support for--the proposition that school factors play an elevated role in generating the black/white achievement gap while non-school factors primarily drive social class inequalities. These findings help explain why black/white achievement disparities grow mostly during the school year (when schools are in session and have their greatest impact on students' learning) while class gaps widen mostly during the summer (when school is out of session and non-school influences dominate). I conclude by discussing the implications for future research, especially as they pertain to what appears to be the most important contributor to the black/white achievement gap: school racial segregation. (Contains 7 footnotes, 4 tables, and 2 figures.)