This study explores whether a school-based group counseling program for adolescent girls, implemented at scale, can mitigate trauma-related mental health harms. In a randomized trial involving 3749 Chicago public high school girls, we find that participating in the program for 4 months induces a 22% reduction in posttraumatic stress disorder symptoms and find significant decreases in anxiety and depression. Results surpass widely accepted cost-effectiveness thresholds, with estimated cost-utility well below $150,000 per quality adjusted life year. We find suggestive evidence that effects persist and may even increase over time. Our results provide the first efficacy trial of such a program specifically designed for girls, conducted in America's third largest city. These findings suggest the promise of school-based programs to mitigate trauma-related harms. [This paper was made possible by the support of the AbbVie Foundation and Paul M. Angell Family Foundation.]