She doesn't need to lose any weight. She needs to grow into her weight. That's quite an important distinction. Please tell her she does not need to diet or lose weight as this could set a pattern for a lifetime that almost invariably ends up with greater gain than loss. What she needs is to learn how to care for her body with healthy foods and regular exercise.
Tell her she will grow taller and the weight will redistribute, so the important things to focus on: eating healthily, making good choices at lunchtime (why the bloody hell do schools get so militant about packed lunches, then serve utter crap in their canteens? There shouldn't be a single unhealthy choice on offer inside a school.) And getting lots of exercise. If she does these two things, she is setting herself up for a lifetime of looking and feeling good.
I'd watch some well-researched documentaries with her about UPF and what it does to our bodies - that excess fat, sugar and salt that is designed to make us crave more. (Supersize me was a famous one - about twenty years old now - there may be more recent ones around) Check before you watch anything on youtube though as there will be a lot of influencers trying to flog inessential supplements and you don't want her to go the other way towards the ProAna movement.
Help her make a list of all the healthy foods she really enjoys and suggest she focuses on them when buying lunch at school, so she doesn't feel deprived. Discuss what options are on offer at school, and suggest she takes a good look over the next few days so she gets into the habit of focusing on the better choices.
I might be inclined to send an email to the Head and the Governors asking that they review the food and have a couple of main meals advertised as 'Healthy Choice of the Day' to encourage pupils to pick those over fries.
Can you exercise with her? A 20 min walk every evening, looking at the Christmas lights, a 10 minute HIIT youtube video in the morning before showering, or a 15 minute yoga unwind before bed will all help her feel more in tune with her body. You could give each other mini challenges, like: do 20 squats or plies while cleaning your teeth, or use light hand weights while watching TV.
Another thing that could help is to make a list with her - as long as you possibly can, of all the non-food treats she loves, so that she can learn to reward herself with something other than comfort food if she is feeling down. Could be anything from a cute kitten video or favourite band's hit song online, an episode of a favourite TV show, a magazine, new nail polish or lip balm, face mask, hair accessories.