Learning can be engaged by dialectic, that is, by identifying pros and cons that inhere in propositions, and more generally, by raising questions about the validity of claims. We report here on a classroom case study of dialectical constructivist pedagogy: Students created dialectical analyses of two lectures and four books as core activities in a freshman seminar "Information, People and Technology". We adapted the functionality of Piazza, a free wiki-style question-answer course management infrastructure, and Toulmin argumentation structures to organize and facilitate these dialectical learning activities. In this paper, we motivate this approach, describe our implementation of it, and present interaction log data and content analysis of Piazza debates, and analysis of student self-reflections on learning activity and consequences, to assess issues in this approach, and directions for further instructional design and research.