Decreasing performance in several core subjects among students in Sweden and the increasing segregation of schools are urgent issues in relation to equity in education, which has been a long-term goal in Sweden. The aim of this study is to identify factors in the school environment associated with student performance in PISA in mathematics at different performance levels in the Nordic countries. In order to separate the effects of school-level variables from the effects of student background factors, and to deal with the multistage sampling design used in PISA, multilevel analysis was used in this exploratory study. Based on data from PISA 2003 and 2012, which are the most recent assessments with a focus on mathematics, results have shown that a few school-level factors seemed to be associated with student success, and then mainly among low and medium-performing schools. Overall, school-level factors associated with success (or lack of it) partially differ between countries and over the years. These results have implications for educational professionals since some of the school-level factors identified inhibit potential for change.