Like tons of people already said, practical thing. Taxes and mortgages and homeowning and rent and tbh handyman stuff too.
ALSO, history that isn't Eurocentric and heteronormative. Teach me about different African cultures and teach me queer history. Sure there's sometimes a paragraph or maybe a section on this stuff in public k-12 edu but not nearly sufficient. I shouldn't have to learn this stuff on my own. I shouldn't only be told about white men in history.
ALSO GOOD SEX EX Hesus CHRIST it ain't sufficient to just teach anatomy or abstinence. Tell me about STDs, be explicit about the line between consent and rape (and dear GOD teach kids about consent in all social relationships from a young young age). Teach me queer sex ed instead of pretending everyone in the classroom is hetero cis. Tell me dental dams and rubber gloves exist. Let's talk about communication during sex and say it's okay to have multiple partners as long as you're safe. Stop sex shaming!!!
First of all, everyday life things, like how to comfort others, how to cope with depression, how to make money, how to hold mga sanggol etc As for my own personal school, it would be a To teach French, Japanese or any other foreign language (We already study English so yeah :3)