in your situation... I woud use this ocasion to see how is my ego responding... and try to separate it from mine consciousness. It is great oportunity for your inner grow... it is not You who is hurt, it is your ego, who wants to be nice and pretty to everybody- when you think about that aware it is inposible and silly.
You can't do nothing to ego, it is stupid to fight against him or something... it is part of us like our hand or nose, but you can be aware and conscous laugh at him like to small silly child :D It is part of us made to keep us live in hard situations, and he is woven with fears and cravings.
Responding to this mean girl pushes you in position of fight, or defense or just staying calm and don't respond.... what ever you decide- but do it with your concousnes, not with your ego.
Awarenes helpes me in any posible stupid situation, and I am learning to use it as my dreds grow :D... I am not best student, but I try to remember to do my homework- lol