The present study highlights teacher stress related to student mental health promotion through the relationship between perceived competence, perceived responsibility and negative emotions. Data were derived from a mixed methods design, utilizing three focus group interviews (n = 15), followed by survey research (n = 771) amongst Norwegian K-12 teachers. The results suggest that teacher stress emerges chiefly from a mismatch between feeling responsible for and being able to help students with mental health problems. The data also point to the impact of time constraints in school context. Finally, the findings reveal significantly higher levels of perceived responsibility and negative emotions amongst female teachers, and significantly lower levels of perceived responsibility amongst teachers at higher grades.