School-based adolescent sexuality education (ASE) programmes in urban Nigerian slums are increasingly recognised as critical for improving health outcomes and reducing risky behaviors among adolescents. However, there is limited research on their effectiveness and potential gender disparities. The study employed a mixed-methods approach involving surveys (N=500) and qualitative interviews (n=30) conducted among students, teachers, and parents. Data were analysed using thematic analysis for qualitative data and descriptive statistics for quantitative data. Findings indicate that while overall knowledge about sexual health improved post-programme intervention, there are notable gender disparities in understanding of reproductive rights and access to healthcare resources. The study highlights the need for tailored ASE programmes addressing specific needs of adolescent girls and ensuring equitable access to education and support services. Programme developers should prioritise gender-sensitive curricula and resource distribution, incorporating feedback from all stakeholders including adolescents themselves. Adolescent Sexuality Education, Urban Nigerian Slums, Gender Disparities, Mixed-Methods Research