Adolescents in Cape Town's South African slums are disproportionately affected by mental health issues such as anxiety. A mixed-methods approach including pre- and post-programme surveys, interviews with educators, and focus groups conducted over a year. The programme reduced average anxiety scores by 20% (95% CI: 10%, 30%) in the intervention group compared to baseline data. School-based mental health programmes significantly improve adolescent anxiety levels, warranting broader implementation in similar contexts. Further studies should explore long-term effects and cost-benefit analyses of these interventions. Adolescents, Cape Town, Mental Health Programmes, Anxiety Reduction Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.