"The only way to improve outcomes is to improve instruction."
That's the key takeaway from a big McKinsey study (PDF file) on global education policy. Regardless of country, teacher quality has more impact on student performance than any other factor, including class size and per capita spending.
That's not exactly news. The interesting part of the study is its examination of best practices for improving instruction, both by recruiting better teachers in the first place and by developing the skills of teachers once they've been hired.