Do you think this is right for a teacher to do to a student?

Since you know that this is just temporary until you hand in those ignment, I don't think it much matters. It's not as if getting bad interim grades in sixth grade will hurt you. They won't count for college, and since they will be quite different by the end of the semester, your high school won't even see them. The only problem I see here is that she never tells you the right ignments to do, and you do need to get that straightened out. Otherwise, her license plate is just a nice, peaceful sentiment, and her grading doesn't really hurt you.