Introduction: Anaemia remains a significant public health concern among adolescent girls in Indonesia. Weekly iron-folic acid supplementation is widely implemented; however, evidence from supervised school-based settings is limited. This study aimed to examine changes in haemoglobin levels following supervised weekly supplementation. Methods: A quasi-experimental study with repeated measurements was conducted among adolescent girls in one junior high school. Participants received weekly ferrous fumarate (60 mg iron and 400 µg folic acid) for 12 weeks under school supervision. Haemoglobin levels were measured at baseline and follow-up. Data were analysed using General Linear Model Repeated Measures (GLM-RM), with additional analysis of covariance (ANCOVA) to adjust for baseline differences. Results: A total of 47 participants completed the study. Haemoglobin levels increased in the intervention group (10.8 to 13.2 g/dL), while minimal change was observed in the comparison group (12.4 to 12.9 g/dL). GLM-RM showed a significant effect of time (p<0.001) and time–group interaction (p=0.001). However, after baseline adjustment using ANCOVA, the difference between groups was not statistically significant (p=0.296). Conclusion: Supervised weekly iron-folic acid supplementation was associated with improved haemoglobin levels over time. However, baseline differences may have influenced the observed effects. Strengthening school-based programmes through structured supervision and adherence monitoring may improve implementation. Further research using baseline-adjusted designs is needed to better estimate the true effect of supplementation.