Conversational technologies such as email, chat rooms, and blogs have made the transition from novel communication technologies to powerful tools for learning. Currently virtual worlds are undergoing the same transition. We argue that the next wave of innovation is at the level of the computer interface, and that mixed-reality environments offer important advantages over prior technologies. Thus, mixed reality is positioned to have a broad impact on the future of K-12 collaborative learning. We propose three design imperatives that arise from our ongoing work in this area grounded in research from the learning sciences and human-computer interaction. By way of example, we present one such platform, the Situated Multimedia Arts Learning Lab ["SMALLab"]. "SMALLab" is a mixed-reality environment that affords face-to-face interaction by colocated participants within a mediated space. We present a recent design experiment that involved the development of a new "SMALLab" learning scenario and a collaborative student participation framework for a 3-day intervention for 72 high school earth science students. We analyzed student and teacher exchanges from classroom sessions both during the intervention and during regular classroom instruction and found significant increases in the number of student-driven exchanges within "SMALLab." We also found that students made significant achievement gains. We conclude that mixed reality can have a positive impact on collaborative learning and that it is poised for broad dissemination into mainstream K-12 contexts.