Purpose: Most studies on the impact of school culture focus only on teachers' average perceptions and neglect the possibility that a meaningful increment to the prediction of school effectiveness might be provided by the variance in teachers' culture perceptions. The objectives of this article are to (a) better understand how teachers' collective decision making and principal's leadership are related to the schools' culture homogeneity and (b) test the moderating role of the schools' culture homogeneity on the relationship between four cultural scales and the schools' mathematics achievement. Method: The sample of this study consisted of 2,595 students nested in 52 schools and 125 classes from the French-speaking community of Belgium. Students performed two mathematics achievement tests and answered a self-reported questionnaire on their backgrounds. All the teachers (n = 817) of the 52 schools answered a self-reported questionnaire dealing with the principal's leadership, school culture, and teacher collegiality. Hypotheses are tested using correlation, regression, and hierarchical regression analyses. Research Findings: First, it was found that culture homogeneity is positively associated with (a) the principal's transformational leadership (TL) and (b) the teachers' collective decision making relative to pedagogical aspects. Second, it was shown that the impact of TL on culture homogeneity is partially mediated by teachers' collective decision making. Third, the analyses found no moderation effect of culture homogeneity on the relationship between cultural values and students' achievement but did show partial evidence of such an effect in specific groups of schools (with low or high socioeconomic status composition). Implications: This study provides a deeper understanding of how social arrangements and leadership processes within schools contribute to the emergence of collective associations between cultural values and student achievement, even when considering an appropriate definition and conceptualization of the school culture. (Contains 10 tables, 2 figures and 3 notes.)