Results from the first study of the three-level School-Wide Positive Behavior Support model in Europe (SWPBS, called N-PALS) are presented. Using a strengthened nonrandomized design, data was collected from more than 1,200 teachers and 7,640 students at four measure points over four school years in 28 Norwegian intervention schools and 20 controls. Multilevel analyses revealed significant positive main and differential intervention effects on student problem behavior and classroom learning climate. Moreover, the number of segregated students decreased in the intervention group, while it increased in the control group. Implementation quality moderated the outcomes. Study results, limitations, and future directions are discussed.