Stress and anxiety are on the rise in children and adolescents, which may adversely impact their social and emotional development, learning, mental health, level of functioning, and educational success. Compounding this issue is that teachers often lack the preparation to best meet their students’ mental health needs. These associated factors constitute the problem of practice that prompted this action research study, whose purpose is to examine the effectiveness of Stress on Students (SOS)—a series of professional development modules designed to educate teachers on student stress and anxiety. SOS was developed with input from teachers through previous cycles of action research. The modules focus on identifying stress and anxiety among students and intervention strategies to increase teachers’ knowledge and perceived levels of self-efficacy. This study was grounded in the theoretical frameworks of andragogy and self-efficacy theory and employed a concurrent, mixed-methods design. Data were collected through a quantitative pre- and post-test survey instrument and qualitative semi-structured individual interviews. Analytic strategies included paired samples t-tests, descriptive statistics of the pre- and post-test, and multiple coding cycles of the individual interviews. Triangulation of the quantitative and qualitative data confirmed SOS’ effectiveness on teacher participants (n = 6) and provided complementary evidence. Teachers showed an increase in their actual and perceived knowledge about student stress and anxiety post-SOS with similar results pertaining to their perceived levels of self-efficacy in working with students who exhibit stress and anxiety. Additionally, teachers fully participated in SOS and deemed the topic and content to be relevant and valuable.