In the class feed, posts which I haven't read yet are marked in bold text. However if I read a post, and then someone later adds a new answer to the question, the post is not shown to me as unread.
I think it would be helpful to mark posts that have new answers since the last time you have viewed them as unread. Otherwise, we would need to re-read all previous posts to check for additional answers.
I have used Piazza previously, and Piazza had a very useful view where you could see any posts that were modified in any way since the last time that you looked at it.