Anxiety disorders among Nigerian adolescents are prevalent but often underdiagnosed and undertreated. There is a need for effective school-based mental health interventions to address this issue. A quasi-experimental design was employed, involving 20 schools randomly selected from urban and rural areas. Participants were assessed using validated standardised questionnaires at baseline (T1), post-intervention (T2), and follow-up (T3). Significant reductions in anxiety symptoms were observed between T1 and T2 among participants, with a mean reduction of 25% (95% CI: [18%, 32%]) on the State-Trait Anxiety Inventory. The school-based mental health intervention demonstrated promise in improving emotional well-being and reducing anxiety symptoms among Nigerian adolescents. Further research is needed to validate these findings and explore long-term effects. Policy makers should consider integrating evidence-based school-based interventions into existing educational frameworks, with a focus on underserved urban and rural areas of Nigeria. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.