Based on national test scores, what we know works in a small homogeneous country at least is professionalizing teaching - raising the standards to become a teacher. They didn't raise salaries to the same level as other professionals like lawyers and doctors, it's about the same pay as the USA, but Finland managed to go from worst to the top in Europe in one generation.
https://www.theatlantic.com/national/archive/2013/01/the-cas...
It's also interesting to note that children in Finland got a shorter school day, and mostly no homework at all. The shortening of the school day was especially prominent in for the youngest children.
