Most educational theorists now promote activity in context and authentic activities to engender more meaningful forms of learning. Problem-based learning (PBL) is one of such pedagogy whereby students work in groups to solve “real world” problems. However, when these practices from the real world are introduced into classrooms, school teachers could encounter many challenges, such as curricula and individual constraints. In this ethnographic study, we describe what happened when a high school physics teacher adopted PBL in his classroom in an attempt to move toward inquiry-based instruction. Using cultural-historical activity theory, we compared his instructional activities with a referent PBL model derived from literature, so as to surface the tensions and contradictions in the activity system as he introduced new practices into his classroom. We found that the challenges he faced arose from disparities between the motives driving everyday practices and schooling, which we attribute to differences between academics and the lived-realities of practitioners. We suggest that researchers work collaboratively with teachers towards an equilibrium point. This joint reflective practice could potentially enable authentic pedagogy such as PBL to be implemented meaningfully and realistically in an Asian society that has long placed a premium on academic achievement.