Adding student collaboration to an intelligent tutoring system could leverage the benefits of both approaches. We have incorporated a mutual peer tutoring script, where students of similar abilities take turns tutoring each other, into the Cognitive Tutor Algebra. In this paper, we identify three design principles for peer tutoring, and discuss how they were realized in our peer tutoring script. We then develop a cognitive model for peer tutoring, and drawing from student data, identify places for an intelligent tutor to provide feedback. Finally, we describe the implementation of the script and our plans for formal evaluation.