Academics is Groupthink?

I recently got back a paper in my English Lit class (which is about the Modern period), and I realized something. There is no such thing as freedom of expression anymore in academics. The reason why I say this is simple:

My paper got a bad grade. Now, this may seem like I'm just doing the typical college-student complaining, but there's something more to this whole story. I talked to my professor after class about why my grade was poor. She said that, get this, I was marked down because my opinion on the story didn't agree with hers. Now, to some of you, this may be a familiar problem, but I was personally flabbergasted. Whatever happened to the "multiple interpretations" she talked about in class?

Furthermore, she told me to look up a "proper reading" of the story. Her words, not mine. What does she consider a proper reading? Are the only "proper readings" those that agree with her opinion?

I spoke to my mother about this afterward, and she told me to go to the dean of the department. I was about to, but, lo and behold, I went to talk to her today, and she marked me back up. Why? "Because the paper is well-written and self-consistent." Well, gee, shouldn't you have done that in the first place, instead of giving a student a bad grade just because his opinion differs from yours?

I suppose I should just be happy that I ended up with a "B" on the paper, but I can't. Not in this clear case of forced groupthink, where you get a penalty if your opinion falls out of line with everyone else's. Where has freedom of expression gone?