tbh I think it depends what the other girl has been telling her parents that has led them to go into school.
If the other girl has gone home and told parents that she is being bullied by this group of girls, then the parents do have a right to go in and talk to the teacher about it. If I thought my child was being bullied then I would go in and talk to someone, and I would want them to be keeping an eye on the situation.
Let's not forget that the girl has made the alagation against your dd and her friends, not the other way around. So the school have no reason to believe that the other girl is unpleasant as well (if she really is and if the other children aren't just saying that to justify their treatment of her.)