Somthing similar happened to me. How old is she?
Reasons why it happened to me... I had an eating disorder prior (did a 'skinny' sport and then gave it up) and then I started eating 'like other girls' and that really hit me. I was also quite stressed, big lifestyle change from being active to suddently having to study hard and stay in more, so had a big bowl of cheesy pasta for stress relief as lessons finished, and then snacks to help with studying as the evening went on. Was also surrounded by people that throught junk food was normal food.
Similar happened to my cousin when she changed schools and had a big lifestyle change (harder academics, loss of full-on sport routine).
Similar happeed in first year of uni as well, again, similar factors.
You're right that if the school is creating an obesogenic environment and think that is normal, that is probably not the place for her. Environment and habits are so important as a teen.
Could she live at home?
She needs to find other ways to manager her stress. A step away from eating out of boredom/ stress, and a move to use physical activity and outdoors time to help with the change instead would be good.
How she learns to cope with changes and environments now is probably pretty key to her future health.
People may slate you for it, but there's actually nothing wrong with being in a more health conscious, active lifestyle oriented environment. Teens eating junk food is not normal, no matter what popular culture tries to tell us.