BFF PROBLEM i need advice!!!!?

Hi. so the story is I did really bad on a math test so my teacher let me fix some probelms after school. And I cant tell my bff that bcuz she will think im bad at school etc. Of course, being the nosey person is she is mad at me bcuz I wont tell her what i did. I dont want 2 lie but its mean of her to get mad at me 4 not telling her my personal stuff 4 school. Advice on what 2 say????