Adolescent girls' capacity to lead healthy lives and perform well in school has been hampered by their lack of awareness about menstruation and the requirements for its hygienic management. Lack of enabling infrastructure, improper menstrual supplies, and limited socioeconomic support for good menstrual health and cleanliness are characteristics of schools in Africa South of the Sahara. We evaluated school-age girls' knowledge of menstrual hygiene and identified bottlenecks that could affect policy and programming for menstrual health and hygiene. A school-based cross-sectional study involved 8,012 adolescent school girls in the age group of 11-18 years (mean age = 14.9 years). The study evaluated students' knowledge of menstrual health and hygiene (MHH) from the viewpoints of schools and communities using a combination of qualitative and quantitative approaches. Data was collected using self-administered surveys, focus group discussions, in-depth interviews, and site observations. Girls' older age (AOR = 1.62, P 0.001), having a female guardian (AOR = 1.39: P = 001), and having a parent in a formal job (AOR = 1.03: P 0.023) were positively associated with Menstrual health and Hygiene Knowledge. MHH knowledge levels varied significantly between girls attending government (53.3) and non-government schools (50.5%, P = 0.0001), although they were comparable for girls attending rural and urban schools. Only 21% of the study's schools had at least one instructor who had received training in MHH instruction for students. We have established that the majority of adolescent girls in schools have inadequate knowledge on menstrual health and hygiene, and that school teachers lack the skills to prepare and support young adolescents as they transition into puberty. Concerted actions aimed at building supportive policy are paramount, for school-aged teenagers to learn about and reap the long-term advantages of good menstrual health practices.