This paper investigates the impact of three different approaches to establishing School Self Evaluation (SSE) mechanisms upon student achievement. Using group randomisation, four groups of schools were created. Different types of support were provided to the first three groups of schools in order to help them establish SSE mechanisms, whereas no SSE mechanism was established in any of the schools of the fourth group. In the first group, school stakeholders were offered the opportunity to develop their own SSE mechanisms and design their own improvement strategies. The second group followed the same process in designing SSE mechanisms as the first, but before introducing this approach support was offered to the stakeholders in order to address and reduce their concerns about SSE. The third group was asked to develop SSE mechanisms and take decisions for their improvement strategies which were in line with the knowledge base of educational effectiveness research. All three experimental groups had better results than the control group, but the impact of the third approach on student achievement was higher than the impact of the other two approaches to SSE. Implications for research on SSE are drawn. (Contains 3 tables.)