Another perspective you need to add into the situation is that 6th grade girls are very defensive when a new perceived threat enters the class. It is also the start of hormonal changes so there is a lot of snarkiness on all sides.
Rather than pushing your DD to answer more specifics about the incident ask her about the girl she is having problems with. What is her personality like, how does she behave in class, is she a good student, and popular or is she a loner and having trouble in class. If she feels like your DD took her place in the social dynamics of the room, there will be trouble all year long. Similarly, if she feels that it isn't fair that your DD is fitting in so well and doing so well then she could be reacting due to jealousy that she isn't able to be as successful. Also,she maybe a gifted girl who decided she needed to hide her brains to fit in and now sees that it is possible to be smart and not have to hide and she is upset with her decision. Middle school is really tough for all girls, but especially smart ones.
Good luck.

Last edited by Kerry; 01/25/16 07:39 PM. Reason: extra word