Replying to My Mom is Stubburn

so on friday we have a half day. My friends and I were planing on skipping the half day and going skiing. The half day we are not doing anything academically. We are getting talked about drug awarness. I got a 70 on a science test after I had studied with the teacher 1 on 1 for 2 1/2 hours. My mom just says, "you can't ski on friday because your grades aren't good. Meanwhile I still have a 89 in science. I am pissed. My mom won't budge, and I think it is ridiculious we aren't doing anything academically.