This is very worrying, CM. We had a very similar situation last year, with our daughter who is now about to be 16. She seemed to lose a lot of weight very quickly, and I got myself into such a state about it. Luckily my older daughter was able to keep an eye on her and report back to me her eating habits at school etc so I was aware that she was eating, albeit small amounts. Things finally came to a head when she started fainting at school, so the importance of breakfast was kind of drummed in to her by the school matron, GP and ourselves. We don't have a lot of time in the morning, so a large breakfast is out of the question, but we soon agreed on things like cereal bars . In fact I have turned her into an incredibly fussy eater by making sure we always have things she likes to eat in the house. She used to eat everything put in front of her, but no more
Also worth noting, are her friends very thin? My daughter's friends are all tiny compared to her and I think she became self conscious about her size, so without making a deliberate attempt to lose weight, she stopped eating quite so much as she was before. Girls at this age are very funny, and so caught up by what they look like. I also thought that Ms Cole on the X Factor was doing her no help as a role model either
However it must have been a phase, because she has now put all the lost weight back on
My youngest got a Wii fit for his birthday and this calculates BMI for you - I was afraid hers would be schockingly low, but actually it was fine. As long as we have all the things she likes to eat in the house, she is eating them.
My advice to you is to try and entice your daughter with something she can take with her on the bus or to school, and try really hard not to make a fuss (the hardest bit), but keep a watchful eye on her. I'm sure like our daughter it is just a phase teenage girls go through but it is still a big worry.
Hopefully everything will sort itself out.