It's heart-breaking when your child is being bullied. How to help her depends partly on what form the bullying takes. In some cases, I recommend taking the child out of the school. Bullying can be that serious. But I can't tell from your letter how serious the situation is.
I would advise working on a three fronts at once.
1. Bully-proof your daughter. Here is a whole article about how to do this, including self esteem, role plays, what to say to the bully, etc.
2. Support her in working out her feelings about having been bullied, which otherwise she will carry. Here is an article about doing that. It is describing a younger child, but the process is the same. Basically, you want to play games with her to help her surface the feelings, help her cry and rage. These feelings will keep her feeling powerless and frightened if they are not expressed. Getting them out will help her claim her own power.
3. Call the school. They don't always help, but sometimes they do. School intervention almost always really helps the situation. That's because the bullies are just kids, also, and they are testing the limits. It is very important that the school sets up clear expectations that bullying will not be tolerated. And if they can go so far as to teach an anti-bullying curriculum, it has been proven to make a huge difference.