Had a similar experience when I was in highschool with an answer on a physics test. She insisted that her math was correct, and pretty much the whole class got the answer incorrect. Despite us telling her why she was wrong, she refused to listen.

I took the problem to a technology teacher I had, who was known for being a extremely intelligent-abrasive-and-fair (was a nuclear engineer before changing career paths and genuinely enjoyed teaching). Explained the situation and gave him the problem to independently solve. He said he didn't want to overstep his bounds with the physics teacher, but the next day our scores had curiously changed to reflect the correct math.

Some people have too much pride to admit fault, especially to those they view as inferior.