Not eating much

Hiya, I have looked at the advice and guidance but it just doesn't explain much. My daughter, 15, isn't eating much and I'm not sure why. Usually it's because she isn't happy and is in a low mood but she is happy at the moment. We have moved forward a step in school and she has ordered a lunchbox and has a ham wrap which is made for her lunch. At the moment she will have it with her but won't touch it but it's a big step forward and it's there if she wants it. She doesn't like eating in school or around other people. She can just about cope eating with me, or her step-dad alone. 

We know she won't eat crunchy food in school but will at home. Anyways, she is eating very little. She doesn't have breakfast in the morning because she doesn't fancy the cereal or croissants she chose, and also because the builders are in the house. She doesn't eat in school, we may be lucky if she has a biscuit. She may have a small snack like a cake bar or yogurt. Then it's dinner and she doesn't tend to have it or wants it later but doesn't. So at the moment she is eating something very little and maybe a dinner if we are lucky. We don't know why and we are trying to get to the bottom but I was hoping you could see this from a different point of view and suggest something that may help. 

We are still waiting for her to have dinner. She's had a packet of crisps and a yogurt but that's it. I let her help do the shopping and pick things but the last few weeks she's barely touched it.