The ability to debate on a topic; that is, the process of generating and evaluating claims, is a core competency for both academic success and civic life. Students traditionally learn these skills by participating in classroom discussions, during which they share, justify, and challenge each other's ideas. However, such discussions may be rare in classrooms and challenging for teachers. Without proper facilitation the discussion can get off track and even become heated. Unfortunately, teachers lack opportunities to improve these important facilitation skills. Coaching is an effective strategy for helping teachers learn facilitation, but it is hard to deliver at scale due to the lack of qualified coaches, time constraints, and teacher discomfort with being observed by a human coach. This project will address the challenges of teacher professional development by designing a simulated classroom environment in which elementary school teachers can practice facilitating discussions with AI-driven student avatars. The system will offer multiple low-cost and low-stakes opportunities for teachers to practice facilitation and receive expert-informed feedback from an automated coach. This project will provide a much-needed solution to one of the most pressing issues confronting education today: scaling up effective pedagogy that fosters students' skills. To this end, this project will integrate research in artificial intelligence, argumentation, and teacher learning to create a nov