It appears the student went directly to the department head, not the teacher, so I don’t think s/he can know whether or not the error was intentional. If they had approached the teacher first and the teacher refused to change it, that would be an indication that it was done on purpose. But I have to wonder why a teacher who wanted to purposely downgrade a student 25 points would choose to do so on only one assignment instead of spreading it out so it would be less noticeable, and why they would do it in such a way that it could be so easily caught by someone else.
This does make me wonder what is going on. I don’t see evidence of intent. Is it possible, OP that you and your family are too upset about the one grade on your transcript that wasn’t an A, and you are trying to blame the teacher?
No matter the cause or the class, you will probably learn more from this experience than you might have in the classroom. Getting a B in a class is not the end of the world, especially when you have had hardships. There may, of course, be financial repercussions as in merit scholarships you no longer qualify for, or class rank implications, etc. But it might be better in the long run to learn how to deal with disappointments.
MODERATOR’S NOTE
Closing thread. The OP has received enough solid answers to go forward, and at this point, the thread is now running the risk of devolving into “He said, she said.”