We created a quiz-based intervention to help secondary school students in Cameroon with exam practice. We sent regularly-spaced, multiple-choice questions to students' own mobile devices and examined factors which influenced quiz participation. These quizzes were delivered via either SMS or WhatsApp per each student's preference. We conducted a 3-week deployment with 546 students at 3 schools during their month of independent study prior to their graduating exam. We found that participation rates were heavily impacted by trust in the intervening organization and perceptions of personal security in the socio-technical environment. Parents also played a key gate-keeping role on students' digital activities. We describe how this role - along with different perceptions of smartphones versus basic phones - may manifest in lower participation rates among WhatsApp-based users as compared to SMS. Finally, we discuss design implications for future educational interventions that target students' personal cellphones outside of the classroom.