In my opinion, the problem with high school is due to the large amount of wasted time. So many classes had dozens of days if not the large majority of the time spent on nothing or busy work. Busy work is a large part of bureaucratic organizations where most of the cogs will end up, but it shouldn't be the goal.
I think many high school courses could be pushed down into earlier education and a great deal of knowledge a university provides could be offered there.
I think some schools are adopting new models, but when I graduated from high school <10 years ago, all but a handful of classes were two semesters long (i.e., autumn and spring would be the same class) and all classes met 5 days a week for approximately an hour, with 7 classes a day. With 180 days of school, with most classes meeting every day of that (save exam time), that is over a thousand hours spent per class.
I think many high school courses could be pushed down into earlier education and a great deal of knowledge a university provides could be offered there.