Chapter 5 BPR/ Discussion

As a teacher I am always looking for new ways to start discussion in my classroom. I think discussion is pivotal for learning and understanding social studies. I have employed many different types of discussion such as whole class, fishbowl, and Socratic seminar. I view myself as a moderator by posing questions that direct the students to particular issues or gets them to explore issues more in depth. SO in essence I agree with the statement in Chapter 5 that says the teachers role in a discussion is to keep it organized and keep it flowing. However, I do have a problem with creating a rubric for discussion. How do I grade someone in a discussion. Traditionally, I have just graded for participation, but would it be possible to grade based on the quality of each students contribution or would that alienate some students from participating?