I have never heard of one student being allowed to retake an exam. That would be entirely unfair to every other student in your school who takes that course. I would be irrate as a student or parent of a student if the teacher allowed you to do this. A bad test grade happens to almost everyone. My son got a zero on a quiz when he read the essay question wrong so did not answer the question but what he thought was the question and it dragged his grade down six points. Its life. No one asked for a retake as it was his error.