Using data from the Early Childhood Longitudinal Study-Kindergarten Cohort (ECLS-K)--a large, nationally representative sample of US elementary school students, we employed multilevel analysis to answer the following research questions: (a) Does students' mathematics achievement growth in grades K-3 vary among schools? (b) To what extent does school academic and disciplinary climate explain variation in mathematics achievement growth among schools? (c) To what extent do students' and schools' demographic characteristics explain this variation? While previous studies have examined the effects of school climate on student achievement in middle school and high school, the present study is focused on the effect of school academic and disciplinary climate on students' mathematics learning in the first 4 years of schooling--from fall of kindergarten to spring of third grade. We found that students' mathematics achievement growth varies significantly among schools and that students' improvement in mathematics achievement over time was higher in schools characterized by a stronger climate, above and beyond students' and schools' demographic characteristics. (Contains 4 tables.)