11 Junior High School District (JHSD) clusters in Chiba City receive a multi-level, multi-component intervention targeting individual, environmental, and community factors for childhood obesity prevention.

11 Junior High School District clusters in Chiba City serve as a comparator, continuing usual practices without new obesity prevention interventions from the study.

The CBH COPI intervention applies multi-level strategies to improve children's environments and foster healthy behaviors. In schools, activities include schoolyard enhancements (e.g., hopscotch), inter-school sports events, nutrition lectures, healthy snacks, and educational materials linking health and academics. In parks and roads, improvements include cycling/walking promotions, signage, healthier vending machines, and community events. Supermarkets support healthy choices through menu information, loyalty incentives, cooking classes, product placement, and healthy food sales. At home and in communities, the Min-Challe app supports habit formation, with athletes, mascots, and ambassadors promoting healthy living. The coordinated program aims to naturally promote healthy eating, physical activity, and sustained lifestyle change among children aged 6-12 in Chiba City.

The primary outcome is the BMI z-score, calculated using the WHO 2007 Growth Reference. This metric standardizes a child's relative weight status by adjusting for both age and sex, enabling comparisons across different age groups and sexes. It represents the number of standard deviations a child's BMI is from the mean BMI of a healthy reference population. The BMI z-score allows for monitoring obesity-related changes over time and assessing the intervention's effectiveness in preventing excessive weight gain. The use of this z-score ensures consistency with international standards, facilitating global comparison of childhood obesity trends. The analysis will focus on changes in BMI z-score from baseline to 1-year and 2-year follow-ups.

HRQoL will be assessed using the KIDSCREEN-10 index, a validated, self-reported measure capturing children's subjective well-being across physical, emotional, social, and school functioning. This 11-item scale uses a 5-point Likert response for each item, which is then converted into a standardized T-score, further transformed to a 0-100 scale. On this scale, higher scores indicate better perceived quality of life. Changes in KIDSCREEN-10 scores will be tracked from baseline through 1-year and 2-year follow-ups to evaluate the intervention's impact on children's overall well-being.

Physical activity will be assessed using self-reported daily minutes of moderate-to-vigorous physical activity (MVPA) and sedentary screen time. Participants will report time spent in activities such as sports, cycling, or active play, as well as screen-based sedentary behaviors. Data will be collected at baseline, 1-year, and 2-year follow-ups to evaluate the intervention's impact on physical activity patterns and sustained behavioral changes. Interpretation will be guided by international guidelines on daily activity targets.