BACKGROUND Given the dramatic increase in adolescent overweight and obesity, models are needed for implementing weight management treatment through readily accessible venues. We evaluated the acceptability and efficacy of a school‐based intervention consisting of school nurse‐delivered counseling and an afterschool exercise program in improving diet, activity, and body mass index (BMI) among overweight and obese adolescents. METHODS A pair‐matched cluster‐randomized controlled school‐based trial was conducted in which 8 public high schools were randomized to either a 12‐session school nurse‐delivered cognitive‐behavioral counseling intervention plus school‐based after school exercise program, or 12‐session nurse contact with weight management information (control). Overweight or obese adolescents (N = 126) completed anthropometric and behavioral assessments at baseline and 8‐month follow‐up. Main outcome measures included diet, activity, and BMI. Mixed effects regression models were conducted to examine differences at follow‐up. RESULTS At follow‐up, students in intervention compared with control schools were not different in BMI, percent body fat, and waist circumference. Students reported eating breakfast (adjusted mean difference 0.81 days; 95% confidence interval [CI] 0.11‐1.52) on more days/week; there were no differences in other behaviors targeted by the intervention. CONCLUSIONS While a school‐based intervention including counseling and access to an after‐school exercise program is theoretically promising with public health potential, it was not effective in reducing BMI or key obesogenic behaviors. Our findings are important in highlighting that interventions targeted at the individual level are not likely to be sufficient in addressing the adolescent obesity epidemic without changes in social norms and the environment.