My son is 8 hehas a mouth on him too! I have watched him learn to cope in different ways over the years.
* Before being verbal it was biting
* Then hitting
* Now sassy mouth
* arguing
He debates me ALL the time! ALL THE TIME! I finally told him NO MORE! It is on his "Uh Oh' box as something to get into trouble for. If I say tha tis what we are doing that is what we are doing. He has told me I suck, etc. I know he does not mean it. It usally occurs when he is frustrated. He is worse with his older brother, stupid, shut up, fatso, goes on and on. Personally I believe it is because he has to hold himself together ALL day at school then he can come home and be himself and let all that frustration out. it is not ok, so it is corrected, but also teaching another way to cope that is appropriate. There are tons of social stories that you can get to help with it too. Fortunately my son stills sees the school social worker, so I tell her when something is going on and she has a better time talking to him. He really respects her. I have to say is don't ever take it personally, we have no idea what is going on inside them. You are her safety net, she can be herself, she loves you and is just this little person dealing with all this stuff inside her. Maybe it is more of something stressful that you do not know about and she cannot express. Is there someone at school bothering her? My friend just told me last night ( her son has aspergers) that the kids at school make fun of him when he gets frustrated, they laugh at him. He told her he wishes he was a normal kid. I know my son still hand flaps a bit, he will tense up and make this noise when he gets too excited. He is trying to find more appropriate ways to express himself in a situation around his peers so not to look different. It is so sad to see, but I am so proud of how far he has come. Then he comes home and he is doing flips on the chair, running, screaming, etc to get all that out he had to hold in at school. lol Maybe play a scene with her with dolls that might take place at school, maybe something will come out of that, ask if she can see the school social worker, Let her know it hurts you when she talks to you like that, ask her if she would like for you to talk to her like that. she is old enough to understand what you are saying to her. Then explain to her it will now be a punishment if she does speak to you disrespectfully, if she is upset about something we need to find another way to express it. get her a pretty journal, maybe she can let out her frustrations in that. This is a time to learn a coping skill for her "mouth". It is inappropriate and she needs to learn how to cope with whatever it is that is bothering her. it will be ok, just another mountain to climb.