I'm not a parent but I totally agree with you and think you have every right to be upset about the situation. The girl should at least be punished to the same degree as your son (in my opinion though what she did was worse, but I guess that's neither here or there). Its true that if the roles were reversed that your son would've gotten into a lot more trouble if he had said that to the girl. I think you need to let the school know that this girl should have her parents called at least to let them know what is going on. Its irresponsible of them not to inform the parents, and by letting her get away with that kind of behavior they are only making it seem like its okay to say that to anyone.
New Clothes Goal:
"What have you done today to make you feel proud?"