The mental well-being of the world's adolescents has decreased in the last 20-30 years. Such a trend is visible also in Sweden, a country otherwise considered a positive example in terms of child well-being. In Sweden, students in lower secondary school are especially exposed. From a salutogenic orientation, this study qualitatively explored 200 Swedish students' (grades 5-9) perceptions of the role of happiness in school. Students perceived happiness as both promoting and being promoted by five aspects: learning, school engagement, appreciation of subjects or lesson content, others' happiness, and prosocial behavior. Hence, five perceived bidirectional crossovers of subjective well-being were found. These were compared to the findings of previous research about the determinants and effects of happiness. The students' perceptions both add new direction for future research and align in several respects with decades of earlier research.