I've had varied ideas regarding running a school -- philosophy, curriculum, etc. -- but I've pinned down the area where I'd concentrate: middle school.
Why middle school? Because that's when people really start to hate school, for one, and because there's so much attention to all other stages of learning. Elementary school education has really improved, I think, and high school education, while not great, gets a lot of attention because it's right before college, etc. But many high school failures start in middle school. College is more than covered by the marketplace. The only other thing I would consider is (older) adult/continuing education, but again -- there's lots of choices out there and adults should be able to take care of themselves.
As well, middle school is about the time that kids can start with abstract thinking, and too often that opportunity is thrown away.
Anyway, that's just a starting point. I've got other ideas, but it helps to focus the mind to think of a target. Middle school.