I would write a letter to the principal. Detail what you have done so far, when you talked to the teacher, the school social worker, and the principal. Let them know that you do not feel as if the issue is being taken seriously. Let them know that they have 1 week to take action to resolve the issue or you will be contacting the district superintendent.
My own boys were the victims of racial slurs and harrassment (some boys were calling them 'stupid honkey,' and 'dumb a$$ white boy' among other things). I immediately wrote a letter to the principal outlining my concerns and hinting that if the school was unable to control the bullying problem, I would contact the district. It was addressed immediately. The very same week, the school had an assembly about bullying and discrimination. The school became a ZERO BULLYING ZONE.
It's important that you not just focus on these two kids, even though it is tempting. Contacting their parents is unlikely to help, since their parents will punish them, and then they'll come to school pissed at your daughter. d Children who bully tend to learn it at home, so getting them into trouble with parental bullies isn't going to teach them not to bully. Just like with my boys' situation, I knew that the kids making those slurs were hearing that sort of talk at home...so there was no sense in the school calling the parents to basically imply that their entire family is a bunch of racists. The problem had to be dealt with it in another way, since it is systemic.
Bullying, discrimination, and harrassment are community problems. By that I mean that, yes, only one or two are doing it...but the rest are standing by and allowing it to continue. So talk to the school about what must be done to make their school a NO BULLYING ZONE. That means that kids who see other kids being bullied don't tolerate it. They tell one another to stop. They stick up for kids who are being bullied instead of supporting the bully with laughs and being bullies themselves. Bullies learn that there's no social payoff for their behavior because THEY become the negative target instead.
Best of luck!
♥
C. Lee