It should be thought of as a professional's opinion, I do...
At school pieces of art work shouldn't be graded/levelled
yes, people should draw whatever they feel like, at home. In art class, the point is to teach you, when they grade you art work badly, they should give you feedback as to why you got that grade, what would have made it better, perhaps if you added color or shadows. You don't need to accept their criticism, but if they grade all ok art pieces a very high grade, no one will learn. It should be thought of as a professional's opinion, I do think the teacher should explain in class that it's her personal opinion and although somewhat professional, it might be that other people disagree with her. She should say that if you get low grades it doesn't mean you are a bad artist, in fact, most really successful people kind of failed in their fields in school. Because there are better and worse pieces, I do think people should be graded, as long as they are given feedback and it's a learning process.