Money management would have been amazing to learn in school. What I really wish I'd been taught in school though is how to code. I reckon my younger brain would have been better able absorb how to write programs.
I'm fairly sure the only computer-related skill I learned in school would be how to use Mail Merge and I have yet to use that a single time since... IT classes would be focused solely on the basics of word processing, spreadsheets, powerpoint, Access etc so I would hope it's a bit different these days with the rise of the Raspberry Pi and so on. Programming has made a bigger difference in both my university and work career than anything else (and I didn't do anything IT related in either) and it's something which not a lot of people have experience in, it's never too late to pick it up.
Besides everything that was already mentioned
1) How to hem a hole in a shirt or sweater.
2) What's dry clean and what isn't.
3) How to wash my clothes correctly.
4) How to save and invest money.
These are all things I've taught myself over the years (except for sowing) that would have been super useful to know.
1) Basic stitching is so ridiculously simple that even I know how to do it, you can learn it in like 5 minutes. I sewed up a hole in my old jeans the other day and I'm fairly sure the part I fixed is stronger than, and will last longer than, the rest of the denim.
2) In my experience unless it's quite expensive (to the point that you wouldn't want to have to replace it) then you can machine wash pretty much anything, even if it says dry clean only, just use a coolish wash.
3) I just put most things in together (excluding suits). Nothing has shrunk or changed colour so far so I'm sticking with it.
4) Learn the power of compound interest, start as young as possible even if it's only a small amount.
I learnt everything I needed in school. *shrugs* Helps that if you're not self-employed in the UK, you don't have to do too much about your tax.
But I learnt to sew, I learnt to cook, I did enough public speaking to get me through interviews... I'm a very high functioning adult really.
You don't really realise how much you learned in school until you actually think about it, do you? I can also sew and cook (how's that for gender equality in teaching), I can change a plug without killing myself, I can fix toilets and other basic plumbing stuff, I can grow plants relatively successfully, I worked for Yorkshire Bank when I was 10/11 (child labour), woodwork, public speaking etc. We actually do a surprisingly good job over here...