The coping power universal (CPU) is an evidence-based universal prevention program delivered by teachers, and completely integrated into the school agenda. Previous studies have shown its positive effects, though little is known about its longer-term effects, and no previous study has explored whether teachers’ occupational stress could influence the CPU efficacy. The current study aimed to explore the 1 year follow up of the CPU on students’ externalizing and internalizing problems and prosocial behavior, and the influence of baseline levels of teachers’ stress in a sample of 316 3rd graders and their teachers (N = 32). Results showed that the CPU led to positive effects, not attainable with the standard curriculum. Additionally, improvements in prosocial behavior persisted even one year after the conclusion of the program. However, improvements in internalizing and externalizing problems were not maintained at the follow up, highlighting the need to understand the factors influencing the CPU efficacy. In this regard, our findings showed that high levels of teachers’ occupational stress predicted poorer improvements following the CPU, and an increase in students’ difficulties at the follow-up assessment. Addressing teachers’ stress as part of prevention programs for students could boost their efficacy and yield more lasting results.