Well Denmark has it easy. It's full of Danes. We have lots of new immigrants and people climbing up from underclasses which don't have a tradition of educational excellence. That part of the solution will take time.
Edit: Let me clarify. Denmark has a small homogeneous well educated population. I would expect that the parents take an intense interest in their children's education resulting in high achievement. In contrast in California we have a much more diverse population with a large proportion of recent immigrants. The parents are not well educated themselves and the kids are struggling because of poor English. This makes it difficult to have a high achieving school system. Sure wealthy communities have excellent schools and can attract the best teachers, but the poorer districts can't. This is not a racial or ethnic problem; it's economic.
Edit: Let me clarify. Denmark has a small homogeneous well educated population. I would expect that the parents take an intense interest in their children's education resulting in high achievement. In contrast in California we have a much more diverse population with a large proportion of recent immigrants. The parents are not well educated themselves and the kids are struggling because of poor English. This makes it difficult to have a high achieving school system. Sure wealthy communities have excellent schools and can attract the best teachers, but the poorer districts can't. This is not a racial or ethnic problem; it's economic.