Obviously, no group can remain productive if everyone is simply arguing for sport. However, groups die even faster when everyone simply accepts a prevailing opinion without challenge. Going back to the original thrust of this thread, I believe that teachers should encourage discussion of controversial political and social issues in class. However, they need to do it in a way that encourages kids to form and articulate their individual positions (not the teacher's position or the most popular kid's position) and then helps them learn how to craft proposals for action that require finding ways for members of a group to work together even though they may disagree on some specifics. This type of team work is the foundation for business, governance, and even healthy families.
The teacher in the video did not do this. It would have been appropriate for her to point out to the girl whose father was in the military that McCain had indicated that we could be in Iraq for many, many years. However, this should have been done to foster discussion of the issues, not to attempt to force the girl to convert to the teacher's position. In the same manner, the teacher should have asked the boy who said he preferred Obama because he was black how he would feel if another child opposed Obama because he was black.