This study analyses an online scenario-based instrument in which secondary mathematics teachers from across the United States were presented with episodes of mathematics instruction and then asked to make a decision at a critical juncture. The theory of practical rationality suggests that the decisions of mathematics teachers can be understood as the enactment of norms, (i.e., the expected actions in the classroom) or the response to professional obligations. Our scenarios explore this theory by providing teachers with the opportunity to breach the norms of instruction and to provide a justification for their choice. We present an analysis of responses from 360 secondary mathematics teachers that provides evidence of the use of professional obligations to justify breaches of norms of instruction in geometry and algebra classrooms. Further, we show how these teachers' willingness to take a non-normative action varies with the nature of the norm and the professional obligation at stake.