Background: Weekly iron and folic acid supplementation (WIFAS) is a key school-based strategy for preventing iron deficiency among adolescent girls, yet adherence remains suboptimal. Nutrition education may improve knowledge, attitudes, and adherence by addressing behavioral barriers. Purpose: This study evaluated the effectiveness of an eight-week school-based nutrition education program in improving nutrition knowledge, attitudes toward iron supplementation, and adherence to weekly iron tablet consumption among adolescent girls. Methods: A quasi-experimental pretest–posttest control group study involved 180 female students aged 15–18 years from two public senior high schools in Padang City, West Sumatra, Indonesia. Ninety students received the intervention; 90 received routine school health services. The intervention comprised weekly interactive education, audiovisual materials, peer-group discussions, educational booklets, and teacher-supported monitoring. Knowledge and attitudes were assessed via questionnaires; adherence was evaluated using weekly monitoring records, supervised supplementation, and teacher-verified consumption. Adherence was defined as consuming ≥80% of prescribed weekly tablets. Results: Nutrition knowledge increased significantly in the intervention group from 61.8±10.5 to 86.7±8.4 (p<.001), versus 62.4±9.8 to 67.2±9.1 in controls (p=.061). Attitude scores rose from 52.6±6.7 to 66.9±5.3 in the intervention group (p<.001), versus 53.1±6.5 to 55.0±6.1 in controls (p=.087). Adherence increased from 46.7% to 84.4% in the intervention group and from 48.9% to 58.9% in controls. Logistic regression indicated intervention participation was associated with higher adherence (adjusted OR=4.73; 95% CI: 2.41–9.28; p<.001). Conclusion: The school-based nutrition education program was associated with improved knowledge, attitudes, and adherence to weekly iron supplementation among adolescent girls.