Purpose: The aim of this study was to examine the means by which principals achieve an impact on student achievement. Research Design: Through the application of structural equation modeling, a mediated-effects model for school leadership was tested, using data from 97 secondary schools in the Netherlands. Findings: The results showed a small positive effect of school leadership on the mean promotion rate in schools, mediated by a development-oriented school organization and favorable classroom practices. The promotion rate may be considered as a measure of efficiency. No indications of direct positive effects of school leader activities on student achievement were found. This might be the result of the relatively small differences in overall student achievement and school leader behavior between the schools studied. Conclusions: The results underline the important role school leaders play in school effectiveness and offer valuable insight in how school leaders actually can make a difference. School leaders were found to have a strong influence on development orientation in schools, which shows similarities with the idea of the “learning organization.” This study points to the importance of school context. The results show that contextual variables have considerable effects on several variables in the model.