Researchers have offered, in recent years, compelling reasons for considering the potential of digital games to support learning and in response policy-makers and educators around the world have demonstrated a commitment to exploring their practical use in school. There remain, however, many questions about how games can best support learning, particularly in formal education. This paper examines the implications of formal game-based learning for teachers developing their own digital learning games, exploring two guiding overarching research questions. What strategies are employed by teachers to manage intergenerational, technological, operational and pedagogic tensions in the classroom? And to what extent is the notion of being a "designer" visible in their professional practice? The paper suggests that there are multiple ways of "being a designer" for teachers, and that the notion of "designer" may be a more problematic representation of teacher agency and identity than currently visible in the literature.