School size and students' achievement. Empirical evidences from PISA survey data

The issue of school size has become prominent for researchers and policy makers alike. Many different arguments have been offered in order to explain how school size might affect student achievement. Overall, if smaller schools are associated with higher students' achievement in primary level, this conclusion cannot be clearly stated for the secondary schools. Empirical evidences highlight that the effect is often mixed: some studies have found higher achievement among students enrolled in smaller schools, while others have detected higher achievement in very large schools, still others have suggested a non linear relationship. In this paper, analyzing OECD-PISA 2012 data, the effect of school size is investigated considering Italian students' achievement, in order to answer the question if there is an optimal school size in Italy. For our goal, due to the hierarchical structure of data (the students are nested within school) we specify a mixed model with random intercept