Schools should teach practical things about living. As it is, it intends to begin preparing people for work, which I think it's premature most of the times. That said, there are things that are really necessary. Take maths for example. Simple arithmetic is a must and algebra should be taught in practical terms. The how to solve real day to day problems should be a must. Schools fail miserably on that, teaching what, but not what for.
Paying bills, taxes, how to deal with neighbors of all kinds, how to choose/buy a car/house, how to get a mortgage/loan and the consequences of not being able to pay it, what are your options to do if you get broke, what is being broke in the first place, some legal things, what is fraud and the consequences on doing so, hazards we could find on a beach setting and how to stay safe, and a long list of things schools fail to teach and yet they are necessary to live on an increasingly complex societal/financial/political system.
More importantly, schools should promote natural curiosity. Little kids are curious by nature and very inquisitive. By the time students graduate, their curiosity is totally dead; the school system had killed it. I'm not sure there is a scientifically proven way to teach in such a way that promotes curiosity; but if there is at least one, it should be the primary approach for schools to take for teaching.