Help with my little sister being bullied?

My sister is 7. Some of the girls in her class are bullies, shouting at her, sidelining her and asking her to do stuff for her. I am so furious. She does stand up for herself but isn't able to do much. She refuses to do stuff for them. The girls sometimes stop other students from interacting with her. We make her understand that she needs to take a stand, we also talked to her teacher. But we can't somther her all the time and talk to the kids in her class. It will only prompt them to exploit her more. I don't know what to do. She's quite shy. Please help. Thanks

P.S. if you don't have anything valuable to say other than nonsensical comments, just don't.