Most upper secondary schools in Norway are so-called sombined schools, where vocational and general departments exist side. This paper investigates whether school characteristics—the number if departments and the size of the academic department—affect the performance of academic stuents. It is shown that student achievement deteriorates when the number of departments increases and that there is an optimal department size dependent on the initial level of knowledge of the average students. The paper takes account of the teachers may be non-randomly distributed among the schools.