The effectiveness of schools is often defined by students’ success in core subjectsand a basic assumption within this research field is that students’ success arerelated to school factors, which can be categorized into context and climatevariables. A key actor is the principal, since he or she is ultimately responsible forthe school and thereby ultimately the students’ success. The overall aim of thisstudy was to identify factors in the school environment that are associated withstudents’ mathematics achievement in the Nordic countries. We focused onSweden and Norway and compared them as they both participated in TIMSS 2015.Effective schools were identified by using average mathematics achievement andconstructed regression models with the student factors and variables as covariates.Schools were regarded as high effective if they were in the top third in theircountry in mathematics achievement, mid effective if they were in the middlethird, and low effective if they were in the bottom third in their country. Schoolfactors were constructed based on the school questionnaire answered by theprincipals. Multilevel analysis was used to separate the school effects from thestudents’ home background. The overall results indicate that few school factorsassociated with student success were found, in particular in Sweden where schoolfactors of importance are mainly related to the student body regardless of thedifferent school types, i.e. low- mid- or high-effective school. School factors ofimportance for student success in Norway, appear mainly to be related to theschool climate. The finding for Sweden is in line with previous studies. However,this is a result that require some thoughts, in the light of the recent development in Sweden regarding the rise of the school segregation and the Swedish long-termgoal of encounter threats to individuals, equal opportunities and access toeducation and future plans on educational investments.