Social robots are employed in many classrooms and have been shown to aid learning. However, studies show that while schools intend for these robots to be social actors, they are not treated as such by the students. As the social factor is crucial for interactional engagement, this paper discusses how students' engagement with a social robot can be systematically investigated and evaluated. For this purpose, I present a small user study in which a robot plays a word formation game with a human, in which engagement is determined by means of an analysis of the ‘language games’ played with the robot.