Theoretical background: More than two-thirds of adolescents were moderately to severely burdened during the COVID-19 pandemic and reported lower quality of life as well as increased stress. Inadaequately regulated emotion and negative stress are risk factors for the development of internalization and externalization problems. Students can use an internet-based app for stress regulation training based on slow-paced breathing. This technique is simple and inexpensive and can be used anywhere. Slow-paced breathing stabilizes, calms, and improves physical and mental well-being. This technique can be used in both primary and secondary prevention. Objective: This study is a pilot project intended to investigate the accessibility, acceptance, and effectiveness of an app with the technology of slow-paced breathing. Method: A multimodule app has been designed and developed that explains the technique of slow-paced breathing and guides the use and practice of this technique before or during stressful situations. An additional breathing app provides support. As part of a pilot project during the second wave of the COVID-19 pandemic, students in grades 6 - 8 tested the 4-week training. 31 students evaluated the entire app, and training with the technique of "slow-paced breathing " was evaluated by 7 students. Results: First results indicate that most users were satisfied with the app technology and other aspects, such as design and processing time. The contents of the different modules were rated well by the training group "slow-paced breathing. " On the other hand, the accessibility was considered low, and half of the students (n = 30) would prefer face-to-face stress regulation training in school. All results are presented descriptively. Discussion and conclusion: The revised app plus face-to-face modules for compliance and motivational incentives should be evaluated in a larger sample of teenage students.