Abstract: Mobile learning is expanding rapidly due to its accessibility and affordability, especially in resource-poor parts of the world. Yet how students engage and learn with mobile learning has not been systematically analyzed at scale. This study examines how 93,819 Kenyan students in grades 6, 9, and 12 use a text message-based mobile learning platform that has millions of users across Sub-Saharan Africa. We investigate longitudinal variation in engagement over a one-year period for students in different age groups and check for evidence of learning gains using learning curve analysis. Student engagement is highest during school holidays and leading up to standardized exams, but persistence over time is low: under 25