Planning, batch cooking and sustainability. If you start with a roast on Sunday, you should have leftovers to make a pie, pasta bake etc on Monday and a soup on Tuesday, even if they don't actually do all the cooking and just think about it theoretically, just get them away from any ideas about only eating the breast of a chicken and throwing the rest away.
Similar for vegetarian food. If you make a big bean and quorn chilli at the weekend, what to do with the leftovers (freeze for another day, put in wraps with cheese for enchiladas etc). Basically, how you can reduce cost, waste and effort by sensible use of leftovers.
Seasonality, budgeting, how own brand or cheaper supermarkets can be just as good as branded, use of leftovers, common sense WRT food storage and use by dates etc.
You see so many posts on here of the 'I left the chicken our of the fridge for half an hour, is it safe to eat' or 'lettuce with use by date of yesterday, will I poison everyone' type variety and I just think where the fuck did this level of risk adverseness and lack of common sense come from because it just seems to be so ridiculous.
The other thing I see so many parents of cooking students complain about (I'm not actually a parent myself but have friends and relatives with school age DC) is having to buy expensive niche ingredients which then go to waste, so encourage either sharing out of bring in ingredients or ask for money to pay for school supplies, so you don't have 30 students all buying lemongrass when 28 of them never use it at home and most of what they buy goes to waste.
Nose to tail eating for meat eaters, there's more to meat than chicken breasts and mince, maybe get them to try foods like pork cheeks or brisket (you'd probably have to cook it at home and bring it in) so they can see how delicious it is, cheap, and more sustainable as it uses all of the animal, not just prime cuts, while being massively cheaper and often more tasty.