Well, I'm a long way from age 12-14, but I might offer a comment or two. For most of my childhood I lived on one meal a day, but I wasn't underweight. I have never been one to eat breakfast and lunches at my schools was horrible at best, especially since I have a great many dislikes WRT many foods. I wonder if part of the problem might be the change of schools, or perhaps the type of food. If it is different from what she was used to previously, then maybe that's the issue. If she isn't eating because she thinks that she's too fat, then it might be wise to seek some professional help.
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"A man needs a little madness...or else...he never dares cut the rope and be free."
Nikos Kazantzakis, ZORBA THE GREEK
Some of us just have a little more madness than others!