Intense debate exists internationally on the study of school effectiveness. A variety of definitionshas been given on this issue along with research findings that try to enrich the theoreticalframework and specify its applications. Among the factors that play an important role in schooleffectiveness, the most important seems to be “students”, “teachers” and “the school principal”.The aim of this study, based mostly on teachers’ opinions and students' reports, is to investigatehow psychological and demographic variables are related with the school system effectiveness .In this research 1671 students and 204 teachers participated, derived from secondary schools inthe broader area of Athens . The specific measurements that were used as the schooleffectiveness indicators include students’ psychosocial adjustment and how much they perceivetheir school as a community that cares for pupils,as well as their school performance andparticipation in school programs, along with teachers’ emotional commitment and openness tochange, their involvement in activities beyond their educational duties and school administration.School effectiveness was revealed in terms of students’ answers that were associated withstudents’ psychosocial adjustment and teachers’ answers towards the perspective of supportiveand with vision management, linked to staffs’ openness to change and associated withperceptions of support, care, respect between parents-teachers and parents-students.