if you really want you can talk to the teacher about what you can do to up that grade and then tell your parents that you got one c on your report card then tell them the things that YOU WILL DO to bring it up. and FOLLOW THROUGH!!! very important.hopefully this helps. it worked with me!
If it was a hard class, then talk to your teacher and do everything you can to make it so you WILL do better next term. They will be less mad if they see that you really ARE trying to do better instead of making up an excuse. Good Luck!
I agree with Isabel and bubblygirl. Speak to your teacher then your parents. If they see that you are working on the situation they may not be as reactive. You may also want to inquire about a peer tutor at school. Ask the teacher if she has extra credit assignments that will help you.