Inform the principle of the school...
Schools take cyber bullying very seriously because of how often it happens these days. I just graduated from highschool this year, and I knew a few kids in a similar situation.
The best solution seemed to be informing the school, they always make sure the kids are in a safe enviornment, and they will even move your daughter to a different school if she wants.
Although bullying usually just passes by time, better to be sure that it stops.