Adolescent anxiety disorders are prevalent in South African secondary schools, where mental health support is often lacking. A randomized controlled trial was conducted with 1500 students from 20 randomly selected schools, evaluating pre- and post-intervention anxiety levels using the State-Trait Anxiety Inventory (STAI). Prevalence of moderate to severe anxiety reduced by 30% in intervention groups compared to controls, with a mean STAI score drop of 15 points. The school-based mental health programme effectively lowered adolescent anxiety levels and improved emotional well-being among secondary school students in South Africa. Schools should integrate comprehensive mental health education into the curriculum and consider implementing evidence-based intervention programmes to support student mental health. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.