Purpose: This study aims to examine the validation of both the model of direct effects and the model of indirect effects of principals' leadership on student academic achievement. Design/methodology/approach: A longitudinal study was conducted in which 22 schools, 55 classes and 1,224 Cypriot primary students participated. Specifically, achievements in Greek Language and Mathematics were assessed at the beginning and at the end of the same school year. Moreover, leadership style of school principals and teachers as well as school and classroom culture was measured. Findings: The findings provide some empirical support for the model of direct effects of principals' leadership on student academic achievement. Moreover, student achievement gains were found to be related with five factors at the school level: the principals' human resource leadership style and four dimensions of organizational culture. At the classroom level, three dimensions of learning culture significantly influence student achievement in each subject. Finally, relationships between effectiveness factors operating at different levels were identified. Originality/value: The article presents an original empirical study which examined the relationship among school leadership, school culture and student achievement in order to validate both the model of direct effects and the model of indirect effects of school principals on student achievement. (Contains 4 figures and 4 tables.)