Make sure that your little girl has an "action plan" for when she is bullied, such as - Telling the bully that she doesn't like what has been said and making sure that she tells the teacher/duty teacher and you when it happens. (Bullies gain power when their victims are silent.)
Have a chat with her classroom teacher just to make sure she is aware of what is happening. Sometimes the bullying happens during the day but kids don't say anything till they've had time to think about it and feel upset. It is best if they talk to the teacher straight away so the negative behaviour can be dealt with immediately.
Ask to have your little girl in a different class to this particular girl next year - usually the classroom teacher is involved in deciding who goes into future classes (remind the teacher about half-way through the last term too.)
Unfortunately some little girls often go through a nasty to each other stage at about 6 to 8 yrs

, but they do grow out of it and learn strategies to deal with the not so nice behaviour.
All the best.