My daughter has experienced 'mean girls' since 2nd grade. The older these girls get, the more subtle their meanness becomes. Teachers always want to make sure that all children are comfortable in their classroom. Don't hesitate to talk with her. Perhaps you could also suggest that the teacher/school start adding some simple bullying awareness to their curriculum. I'm sure there are loads of resources they could draw material from.
One of my sons has a speech impediment and spent several years participating in speech therapy classes at school. He has had kids make fun of his speech, and he sometimes asks if i think people would treat him differently if he didn't have a speech issue.
I encourage you to enroll your child in speech therapy classes asap. The earlier the better, in fact. In our state, Mass., speech is offered in the public elementary schools to qualified children when they turn three years old. Prior to their third birthday, they would receive services from Early Childhood Intervention. These are all low cost services delivered by qualified professionals.
After a few years our Special Education Teacher in the public school determined that my son no longer qualified for services. (His speech problem didn't go away but his grades were too good...i.e., his speech wasn't affecting his grades, therefore he didn't qualify anymore.) I eventually learned that there were other service providers in private practices and through local hospitals or outpatient/rehab centers, esp. pediatric ones. (Your doctor, the hospital and the school system's special education director could point you in the right direction if you choose to go the private route which i would recommend.)
I preferred the private lessons over those at school because they were focused only on him and his unique problem with specific sounds. Our insurance plan covered these appointments and we just paid the co-pay each week. He met one-on-one with the same woman for one full hour every week. At school, he participated in one 30-40 minute class with 1-2 other children each week. They would play games, perhaps a board game and when it was his turn he had to say a word that had the sound he was having trouble with. (There were other things they did that were more intense but I think that was when he was younger.) Clearly, these services were inferior to the ones at the pediatric rehab center. In fact, the spec. ed. teacher said it's frustrating that they couldn't do more with the kids but there were lots of kids who needed services coupled with budgetary constraints.
So there you have it...way more of a story than you probably expected. Best wishes.