What should I do if I want to take hot lunch to school?

I have a HydroFlask food thermos, and it keeps food piping hot for HOURS. Preheat the thermos with hot water before you put your food in there. I love soups/stews, but you could also make a big batch of pasta or hotdish to reheat and stuff in the thermos for lunch.

Get a Yeti thermos. They have these tumblers called "ramblers" that would work very well for soup. I'm recommending specifically the Yeti brand because OMG, their products keeps things either cold or hot for HOURS. I've had hot coffee stay pretty hot for almost 8 hours. At the 10 hour, it's still warm. The Yeti's are on the pricey side but well worth the $$. I don't have any connection to this brand in any way other than a very pleased customer.

Heat up your soup in the morning while you are getting ready to leave, put the hot soup in the rambler, and it should be still hot by lunch time. I have not tried this with other hot foods, but if it can keep liquids hot, then other hot foods will probably work too.

Soup 🍲 in a thermos was included in our lunch box back in the 50’s. Wonderful! You can also fill a thermos with beef stew or chili. Lots of options. If you don’t have time to make soup, there are many good canned options. Heat it and pack it!

Soup (or chili, stew, etc.) in a good thermos as Nancy recommends is a great idea. If you make your own and you are worried about food safety, freeze a portion of it for thawing later in the week and bring it to a boil before putting in your thermos. Do you have access to a microwave? Your idea of batching a large amount of food ahead is a great one so long as you don't get tired of it along the way or make it so you can easily vary it with different accompaniments or toppings. My solution to your conundrum was to make black beans in a crock pot. Start with 5 cups rinsed dry beans, 3 cloves of minced garlic, one onion diced, some chili powder or fresh peppers cored/diced, plus 11 cups water, and cook on low for about 10 hours. Near the end of cooking, add salt (start with 1Tbs) and lime juice and season to taste. If you like smooth beans, use an immersion blender or potato masher to blend them up a bit. Let it cool uncovered in the fridge. When ready, warm large tortillas, add cheese, fresh diced onions, tomatoes, hot sauce, or anything else you want and fold them into burritos. Wrap tightly with plastic wrap and label them with a marker if you use different fillings. This makes about 16 burritos. They're nutritious and cheap. A few years ago, I priced them out at $0.62 each. But they require a microwave at a minimum to heat up. You could get more creative, but that's the easiest.

Select a thermos that will keep food hot for 12-14 hours (not all do).
Heat the thermos a few minutes first with hot water and then empty it.
Pack hot food hot.
Remove leftovers (if any) at end of day, wash and start over

I one bought my little brother a tiny (maybe a 14" cube or so) combination fridge/oven (it switches from one to the other) that can plug into a car lighter socket. Don't remember who made it- I think I got it at Bed Bath and Beyond). If you're driving, perhaps you could use something like that. There are books full of soup recipes; you could probably find a couple of million with a simple search on this site, or any of several others.