I've never seen such a students' walkout or sit in in Japan. For me, it is very, very good for students to think about their politics, to discuss the political/social problems, and to do something related to their society even though it actually doesn't change the society at all.
I welcome walkout and sit in. But you guys should attend the class and ask Prof Mankiw to discuss. This is the first task that you should do(even though I missed class as a student and I have no qualification to give such advice).
Anyway I envy you because here in Japan there are (said to be) too many grass-eating young students( those who don't work hard, have no girl/boy friend, and no hope/dream for their future) . It seems that I can't have anything to expect them to make "the second Japan's miracle" come true.
However, I will struggle to study economics in order to have them think about how their society really works and how they should play an important role in the society.