Is there a reason it has to be a hot lunch? My experience with young children, even kindergarten & first grade, is that they can't really open the containers that have to be super-sealed to keep a hot lunch hot. It's hard enough for them to manage zipper bags and standard plastic containers, let alone a thermos. Soups are messy and difficult for them to manage in a short lunch period. Teachers and aides aren't always there to open every container - or if they are, they have a bunch of kids to help. She's bringing it all home, so maybe she isn't sick of the choices so much as unable to manage them.
I wonder if you'd be better off with a cold lunch - yes, the temps feel "cold" to you and you feel perhaps like you need to warm her up, but if SHE isn't cold, or if she isn't getting enough calories to nourish and warm her through her body utilizing them, it doesn't much matter what you pack. Any food will warm her if she eats it!
I know many schools don't allow peanut butter or other nut butters, but yogurt, cheese cubes, veggies and hummus dip, hard boiled eggs and so on might at least get consumed. Even French toast sticks made with "holey" bread (to absorb the batter) and a ton of egg would pack decent nutrition - I used to make it for my son with a soy/egg mix and a bunch of cinnamon. You can serve your hot soups and stews at night when you are there.