Equitable educational opportunities necessitate equitable access to a favourable academic and social school environment. This thesis aims to explore the role of the school as an educational and social arena for the academic achievement and psychological well-being of lower secondary school students in Sweden. Using survey data, four empirical studies assessed contextual and compositional aspects of schools and their associations with adolescent outcomes.

Study I examined the relevance of indicators of school effectiveness for student academic achievement. Teachers’ ratings of effectiveness indicators and student-reported marks in core subjects were shown to be lower in more sociodemographically disadvantaged schools compared to schools with a more privileged student body. However, even when adjusting for the school’s student intake and students’ own family backgrounds, students performed better, on average, in schools that were rated as more effective. School ethos was found to partly mediate the relationship between school student composition and academic achievement. Furthermore, analyses indicated that students attending one of the most socially deprived schools performed better when their school was rated as more effective, regardless of their own family backgrounds.

Study II focused on school ethos, exploring the mediating effect of students’ academic achievement on the levels of internalising and externalising indicators of poor psychological well-being among students. Teacher-rated school ethos was found to be predictive of students’ academic achievement, even when taking the school’s sociodemographic student composition into account. Moreover, students’ levels of psychological distress were shown to be indirectly associated with the school’s ethos, via academic achievement. No such mediating relationship was identified for students’ levels of aggressive behaviour.

Study III assessed the significance of the sociodemographic school environment for adolescents from socially disadvantaged residential areas in Stockholm. Students who chose to commute to more prestigious schools outside of their residential areas performed better academically compared to their peers who enrolled in one of their neighbourhood schools, an association that was partly mediated by teacher-rated school ethos. However, the commuting students reported lower school satisfaction and more psychological complaints than students who stayed behind, even when taking academic achievement and school ethos into account.

Study IV investigated the implications of classroom immigrant density for adolescents in Sweden who were foreign-born and/or had parents who were foreign-born. Analyses found that students with an immigration background reported fewer psychological complaints when the proportion of immigrant students in their class was higher, an association that was partly mediated by the experience of classmate acceptance.

This thesis has illustrated the importance of the functioning of schools for adolescents’ achievement and well-being, in line with school effectiveness theories. However, schools’ capacity to build an effective school context is closely linked with their student intake, undermining the school’s compensatory mission. Furthermore, the sociodemographic composition of schools can be essential for the psychological well-being of certain groups of adolescents, potentially counteracting the social mixing of students.