It does get easier with time. Be matter of fact and don't let them see your feathers ruffled.
The other day I walked up just as another student was telling a teacher she didn't want to go ask me anything she couldn't stand me. I simply responded with you don't have to like me, you just have to do what you are supposed to do now get back to my room. Unfortunately it embarrassed the child but I bet she will watch herself from now on.
Others have said it and you have to stick to it with everyone you teach...treat them the same. Some kids need special assistance if they are sped...these are sometimes your exceptions. Use good judgement.
You are not there to be their friend. You control you. She can not MAKE you mad, that is your choice. Good luck!