First I would have her checked to see if she is oversuppressed.  This can cause increased appetite, eating more in response, and weight gain.  My daughter's weight problem started after being oversuppressed for a short period of time.  Then she had developed bad eating habits ("sneaking" snacks when we were not there, having seconds of everything at dinner, eating chips, etc when she was at a friend's house).  After the meds were under control, we found that her eating habits were not.  This occurred from the time she was about 9 until just recently and she is now 13.  What happened with her is she developed gastric reflux which made it impossible for her to eat for almost 2 weeks until we could get her on medication to take care of that. (Being overweight contributed to the reflux problem.)  Now Kristni is very careful about how much she eats - and the fat content of everything she eats.  It has made so much difference.  The weight is continuing to come off.  She looks and feels so much better.  As a bonus, I have been eating better myself and have dropped a few.  The whole family is involved in healthier eating and it makes it easier for her as well.

If I were you, I would check first to see if she is oversuppressed and then maybe talk to a dietician to see how you can alter her everyday diet. It's amazing how many good low/no fat foods actually taste good.  I will also tell you that at 13 my daughter is much more self conscious about how she looks than she was at 9 and I think most teenagers will probably be a little more vigilant about their diet if they want to lose simply because of peer pressure.

Good luck.