Purpose: The purpose of this study is to identify predictors of achievement in mathematics in elementary schools in New Brunswick (Canada). Data Collection: Both teachers and school leaders (N = 111) completed a questionnaire on their practices and on school functioning. Findings: Multiple regression analyses revealed that the students' achievement in mathematics was determined by prior achievement, urban school attendance, and teaching quality. Implications: To counter the socioeconomic constraints on achievement in mathematics, taking a closer look at teaching quality represents a promising research area. The implications of this study on education policies and research are discussed.