Having high school education last until the onset of adulthood does have a number of benefits, but to realize those benefits, Americans need to stop treating high school as the first step towards University.
The purpose of government-provided education should be to prepare its students to become citizens.
As a side benefit, those students are also employable, because the gamut of skills that you need as a citizen overlaps heavily with what you need to succeed, either as an employee, or as an entrepreneur: critical thinking, rhetoric, art, logic, practical mathematics, writing and communication, collaboration, physical fitness, maintenance of health and wellness, self-directed learning, a knowledge of history and politics, knowledge of both the sciences and how to apply the scientific method, and probably more than a few more things that I have overlooked.
American high schools teach few of these. Sure, you are required to learn "science", but I don't recall my school ever forcing us to go through the process of creating a theory, designing an experiment to attempt to falsify our theory, and running through the experimental process.
Same goes for critical thinking -- arguing against the teacher's viewpoint is strongly discouraged. You are only allowed to color within the lines.
Physical fitness classes are largely a joke, mostly because they don't cater to things that might be interesting to people that prefer individual sports. I would have loved if my school offered Karate, climbing, or even just hiking as physical fitness courses.
We should also take a page from Japan, and make students responsible for the upkeep and maintenance of their schools, under supervision, of course.
I doubt any of this could be done in public schools at this stage, though -- the liabilities are too great, and there are too many special interests that make a large amount of money off of the current system.