This case study will highlight practical lesson learned from the use of mobile devices with Tangerine®:Class open-source software for data-informed instructional decision-making from a year-long, rigorous study conducted by RTI International in Kenya. Throughout 2013, twenty-one teachers and 600 pupils used Tangerine:Class on Android tablets in a randomized controlled trial. While the trial was focused on early reading instruction, the applicable lessons learned are also for mathematics and independent of subject matter. Practical findings concern training and support, the logistics of individual pupil assessments in large classrooms; the nature of instructional decision-making; and the use of data and interactions between teachers, headteachers and parents. This is a chapter of an edited book "Mobile Learning and Mathematics" (Crompton, H. and Traxler, J., eds.) available at Amazon.com.