The Next Generation Science Standards and other reforms call for students to learn science-as-practice, which I argue requires students to become epistemic agents--shaping the knowledge and practice of a science community. I examined a framework for teaching--ambitious instruction--that scaffolds students' learning of science-as-practice as they act as epistemic agents. Using a situative theoretical framework and analytical tools from science studies literature, I conducted a multicase study of five beginning teachers. I found that (a) teachers and students negotiated their roles as they decided on "what counted" as science ideas. Participants positioned some ideas as important by making discursive moves, signaling students to either work on the ideas as epistemic agents or, alternatively, to judge the information as "right" or "wrong"; (b) the participants worked to make science a "public" or "private" enterprise. The framing of science then influenced how teachers and students participated in their science practice community; (c) the negotiation of "what counted" as science ideas and the framing of science as "public" or "private" influenced (i) the percentage of students sharing ideas on the public plane, and (ii) the number of science ideas initiated and kept in play on the public plane.