Anxiety disorders are prevalent among adolescents in coastal Kenya's schools, yet access to mental health services is limited. A mixed-methods approach was employed, including surveys, interviews, and observational studies conducted in ten randomly selected secondary schools over one academic year. There was a significant increase (p<0.05) in the proportion of students diagnosed with anxiety disorders after participating in mental health support programmes, from 12% to 24%, suggesting improved awareness and early intervention. School-based mental health support programmes effectively enhanced both the detection and utilization of treatment for anxiety disorders among coastal Kenyan adolescents. Continued implementation of such programmes is recommended along with ongoing evaluation to ensure sustainability and effectiveness. Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.