There is increasing awareness of the challenges that young people who do not complete upper-secondary school may encounter. The aim of the current study was to investigate teacher-student relationship as a possible mechanism to reduce the associations between mental health problems, grades, and subsequent noncompletion. Mental health problems and teacher-student relationships were assessed through students' self-reports in 10th grade, and linked with Norwegian registries of education and sociodemography (n = 10,931). A dual-factor serial mediator model was specified, allowing the effect of mental health problems on school dropout to be mediated by the teacher-student relationship via school grades. Results indicated that teacher-student relationship is a potential mechanism to reduce the negative associations between mental health problems and later noncompletion. However, students with mental health problems seemed to experience less supportive teachers; therefore, interventions targeting teacher-student relationships may be required. The patterns were similar between genders.