Even though well-being in the domain of school is important for mental health, time trend research of this topic is limited. In particular, there is a lack of research investigating differences between ethnic minority and majority students, and there is a lack of literature empirically exploring possible explanations for the time trends in school well-being. The aims of this study were to investigate time trends in school well-being among adolescents in Oslo, explore moderations by gender and ethnicity, and uncover potential underlying factors for the possible time trends. This study used data from the Young in Oslo surveys, conducted by NOVA. Time trends of school well-being, and possible explanations, were analysed based on responses from a representative sample of 20,403 students in secondary schools in Oslo from 1996, 2006 and 2015. Analysis of variance was used to test for time trends in school well-being, and identify any differences across gender and ethnicity in these trends. Further, analysis of covariance was used to control for the social relationships to teachers, peers and parents to see if this reduced the effect of time. Finally, linear regression analysis was used to identify changes in the associations between the social relationships and school well-being over time. Results indicated an increase in school well-being from 1996 to 2006, and again to 2015. Ethnicity moderated the time trends, with ethnic minority students reporting higher school well-being than their majority peers. However there was a larger increase among the ethnic Norwegian students over time, resulting in no ethnic differences in 2015. The levels of the social variables changed over time, however not in accordance with levels of school well-being. Nevertheless, the increase in school well-being can be interpreted in light of increasingly strong correlations to relationships to peers. This means that the increase in school well-being was mainly seen among those who had friends, and were never bullied. The increasing correlations between school well-being, friendships and bullying were discussed regarding speculations of higher social demands at school, and increased use of social media. Implications of these results include efforts to address the well-being in school among the proportion of adolescents who are being bullied or have no friends, and to ensure that all students, regardless of ethnic background, will continue the positive trends of school well-being.