Introduction: Teachers can help identify mental health issues in adolescents and act as gateway-providers by referring adolescents at risk to a mental health professional. Studies have so far investigated awareness concerning mental health issues among primary school teachers in the USA. The present study uses case vignettes to examine whether secondary school teachers in Germany can detect and assess the presence and severity of mental disorders in adolescents, and which factors predict referral to professional support services. Methods: N = 136 secondary school teachers completed an online questionnaire with case vignettes depicting students with moderate or severe internalizing and externalizing disorders. We assessed the ability to recognize mental health issues and evaluate the level of severity, worry and perceived prevalence of the problem as well as the helping behaviour among teachers. Results: 66 and 75% of the teachers were able to identify mental health issues in case vignettes of externalizing and internalizing disorders, respectively. 60% and 61%, respectively, designated the mental disorder correctly as externalizing or internalizing, and the true positive rates did not differ between externalizing vs. internalizing disorders. However, moderate and externalizing disorders were identified with less precision, and recommendations to seek professional mental help were more seldom made for these disorders. Discussion: The results indicate that teachers can validly and probably intuitively identify (at least severe cases of) mental disorders in their students. Given the uncertainty expressed and the substantial interest of teachers, further education and training on mental health conditions disorders in adolescents is recommended.