Ban junk food sale in schools. No vending machines, no advertisement, etc. No snacks, candies. Ban colorful attractive marketing to kids, teach basic nutrition in school.

Make school lunch easy to afford, perhaps free and put limits on sugar, salt etc. No pizza and fries, but proper cooked meals. If you don't eat it, you can't get anything else.

Buuut freedom, companies, free markets, economic growth!! Parents would be outraged, kids wouldn't eat it etc. Good luck.