Computer-supported fitness interventions for adolescents have the potential to improve adolescents’ attitudes and perceptions about physical activity through peer influence and interpersonal accountability. Past research has explored the potential of interventions based on competition and social-comparison mechanisms. We present a new approach: school-based, pervasive social fitness systems. We describe one such system: StepStream, a pedometer-based microblog we designed and deployed for four weeks with 42 US middle school students. StepStream users improved their attitudes about fitness and increased their sense of social support for fitness. The least-active students also increased their daily activity. We show that our school-based social fitness approach performed comparably in attitude and behavior change to more competitive or direct-comparison systems. These results expand the strategies available computer-supported fitness interventions. Our school-based social fitness approach to everyday adolescent health shows the potential for social computing systems to positively influence offline health behaviors in real-world settings.