Adolescent girls in Lagos, Nigeria are at risk of poor nutritional status and mental health due to inadequate nutrition education. A longitudinal study was conducted with a sample size of 250 adolescent girls in Lagos, Nigeria. Data collection included pre- and post-intervention surveys, anthropometric measurements, and self-reported mental health assessments. The intervention group showed a statistically significant improvement in Body Mass Index (BMI) scores by $\Delta BMI = 1.2 \pm 0.3$ kg/m² compared to the control group ($p < 0.05$), indicating an effective nutritional education programme impact. School-based nutrition education programmes had a positive effect on adolescent girls' BMI and mental health, with robust statistical support. Further research should focus on sustainability of the intervention and its broader community impacts. nutrition education, adolescent girls, Lagos, Nigeria, longitudinal study