I recommend you either get a stainless 'food jar' or a cryofreeze pack that can be used as a warm or cold pack.
The food jar is a small thermos for food. If you want the food to be hot, boil water first and pour it into the food jar and let it sit a few minutes. Then pour the water out and put the HOT meal in. This will keep it warm a few hours.
The cryofreeze pack/set I have was made by California Innovations. The cryofreeze pack looks like an ice pack but you can out it into boiling water for 4 minutes to use as a hot pack. The set I have came with a rectangle container and then an insulated cover that you zip everything into.
Lately I have been using a California Innovations lunch box that I found at Walmart. It has a velcro section on the bottom that came with a rectangle container. I use it with the hot pack. Up above there is a separate section when I put her water bottles, yogurt, and fruit.
This has been my solution to sending hot foods to school and keeping them warm. Love, love, love California Innovations!!