This study aimed to investigate the effectiveness of a school-based universal preventive stress management training program for early and middle adolescents in comparison with a no-treatment control group. The study examined the intervention effects of age (early versus middle adolescents) and gender on perceived stress, interpersonal coping, and self-efficacy prior, immediately after as well as 3 months after the intervention. Three hundred and twenty adolescents (ages 10-14 years) participated in the study. Whereas both experimental conditions did not differ substantially in baseline scores, the experimental group scored higher on perceived self-efficacy compared to the control group at the follow-up assessment. Additionally, the experimental group showed less perceived stress and more adaptive coping at the post and follow-up assessment. Age-dependent intervention effects suggested that early adolescents primarily benefited from the treatment. Although the effects must be replicated using a randomized design, the current findings reveal that the program does strengthen important protective factors for the psychosocial development of adolescents.