My DD1 stopped eating lunch at school for a while because all her friends were 'on diets' so they would all sit around and pick at imaginary stomach rolls and say how fat they were. DD1 said that if anyone dared to get food out they would pounce on them, analyse the calorie content and look horrified - as in 'oooh, an apple, that's 60 calories oh my gosh I could never eat that, I'm sooo fat' and so on. Apart from the fact that would put anyone off their lunch, DD1 didn't want to be the odd one out.
I was fuming, my DDs are rail thin and do a lot of exercise. They need to eat, regularly or else they get dizzy, faint and shaky. Not to mention no lunch is dreadful for an afternoon of school lessons. But I've learnt that when a group of teens get in that mindset, nothing will change them.
I made sure DD1 had breakfast and an after school snack plus a proper tea. I packed her quick snacks she could eat on the bus or even in the loo if she needed to eat without being spotted. Thankfully the phase passed by Year 11 and now they all eat normally.
DD2 on the other hand is a fussy madam who is one of these odd people who really isn't a fan of food. She doesn't eat because she doesn't want to, and doesn't feel hungry. She's always been like that and its a massive worry. With her, it's a case of I'd rather she ate something than nothing, so would happily feed her chips and cake if it worked. She has a cereal bar on the bus (can't face morning food) and takes nuts, apples, crisps etc to school. A whole meal over faces her and always has, so we try to keep a constant stream of healthy high calories on tap.
I'm not looking forward to the food stage with DD3 and DD4, you have my full sympathy, kids can be a pain!