Its so great to see that you're reaching out for some help on how to help your granddaughter overcome the feelings she has and the bullying she is going through at school. It is never easy to see somebody we care about being mistreated, and bullying is a huge problem in a lot of our schools.
As a caring adult in her life, you have a great chance to serve as a big advocate for your granddaughter. If she won't reach out for help and make the school faculty aware of what is going on, there is no reason you or her parents can't take this step for her. If bullies are presented with a reason to stop (fear of punishment, for example) there is a good chance the bullying will continue until summer arrives or they find a new target. Reaching out for help from the faculty would be a really good step for somebody to take- whether it is her, her parents, or yourself. Remember, you can be a strong advocate for her.
Also, make yourself available to your granddaughter. Let her know that whenever she needs somebody to listen to her you are always just a phone call away. It helps to know that you have support available, especially when somebody is struggling. You need to remember that your feelings are important to, so don't feel like you should be the only source of support for her. Encourage her parents to do the same thing.
kids feel really alone when they're bullied. They feel as if even if they do say anything either no one will do anything about it. Or that she will be called out about it and be teased by others. The best thing to do is to encourage her to say something. & If it comes down to it, go down to the school your self & speak up. Tell them what you know.