This study aimed to estimate the 10-year cost-effectiveness of school-based BMI report cards, a commonly implemented program for childhood obesity prevention in the US where student BMI is reported to parents/guardians by letter with nutrition and physical activity resources, for students in grades 3 to 7. A microsimulation model, using data inputs from evidence reviews on health impacts and costs, estimated: how many students would be reached if the 15 states currently measuring student BMI (but not reporting to parents/guardians) implemented BMI report cards from 2023 to 2032; how many cases of childhood obesity would be prevented; expected changes in childhood obesity prevalence; and costs to society. BMI report cards were projected to reach 8.3 million children with overweight or obesity (95% uncertainty interval [UI]: 7.7-8.9 million) but were not projected to prevent any cases of childhood obesity or significantly decrease childhood obesity prevalence. Ten-year costs totaled $210 million (95% UI: $30.5-$408 million) or $3.33 per child per year with overweight or obesity (95% UI: $3.11-$3.68). School-based BMI report cards are not cost-effective childhood obesity interventions. Deimplementation should be considered to free up resources for implementing effective programs.