One of your first steps should be to set up a meeting with your child's teacher and principal so that you have a chance to speak with them privately about the bullying. During the meeting, discuss with them your concerns about your child's grades. If you feel that her low grades are a result of her being a victim of bullying, then you should ask them if there is a way for her to make up her work or attend summer school so that she does not get held back.
Then seek the advice of your school counselor. They will be able to give you and your daughter advice on how to cope with this difficult situation. If your school does not have a counselor, try contacting a child psychologist in your area for more specific recommendations. I'm attaching a couple of great resources on bullying that will also get you going in the right direction.