Adolescent sexual health behaviors in rural Tanzanian high schools are influenced by various factors including HIV/AIDS stigma and risk perceptions. A longitudinal study design was employed with data collected from baseline surveys in and follow-up assessments every two years until . A mixed-method approach combining quantitative survey data and qualitative interviews was used to assess changes in sexual health behaviors, knowledge, and attitudes. There was a significant decrease (p < 0.05) in the proportion of students who reported condom use during their first sexual encounter from baseline to follow-up surveys, indicating improved awareness and behavioural change towards safer sex practices. The school-integrated HIV/AIDS counseling and testing programme demonstrated effectiveness in enhancing knowledge about HIV transmission and prevention among adolescent populations in rural Tanzanian high schools. Continuation of such programmes with additional support for peer education initiatives is recommended to sustain the positive behavioural changes observed. HIV/AIDS, Adolescent Sexual Health, Counseling and Testing, Rural Tanzania, Longitudinal Study Treatment effect was estimated with $\text{logit}(p_i)=\beta_0+\beta^\top X_i$, and uncertainty reported using confidence-interval based inference.