Abstract
BACKGROUND: Schools play a vital role in promoting healthy behaviours through water, sanitation, and hygiene (WASH) programs, thereby shaping the minds and habits of young people. Urban slums face significant WASH challenges, exacerbating health risks among school children. School-based interventions -WASH in School (WinS) can improve health, promote good hygiene practices, and create a safe learning environment. This study evaluated the effectiveness and sustainability of a school-based WASH intervention in promoting safe WASH practices in urban slums. METHODS: The study was a 4-month post-implementation qualitative evaluation in four purposively selected intervention schools in two urban slums in Anambra State, Nigeria. Data were collected through four focus group discussions (FGDs) with 32 students and twelve in-depth interviews (IDIs) with teachers and school administrators. Data analysis guided by the adapted implementation research outcome (IRO) frameworks, with coding and categorization into themes and sub-themes. RESULTS: The intervention demonstrated positive outcomes across various themes. In terms of engagement, it successfully engaged the target audience, achieving high participation rates, sparking enthusiasm and satisfaction among schools. Regarding effectiveness, the intervention improved knowledge and behaviour related to safe WASH practices, leading to cleaner school environments and perceived better health outcomes. In terms of adoption, schools successfully integrated the intervention into their systems, with support from school administration and provision of necessary materials being key to adoption. The implementation was done with fidelity as schools adhered to protocol/guidelines, although resource and time constraints challenged the process. For maintenance, the sustainability plans include training and retaining of students and teachers to lead WASH clubs, as well as continuous awareness campaigns on safe WASH practices. However, financial and technical support were revealed to be required to maintain these efforts. CONCLUSION: The intervention effectively promoted safe WASH practices and improved health outcomes among urban slum students. To sustain these gains, governments and stakeholders should prioritize integrating WASH initiatives into existing health and education programs. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12889-026-26644-6.