As a former teacher, I would posit that "improvement" should be measured in social measurements, not academic ones. It's infinitely difficult to pin down a standard that measures academic progress for a broad range of socioeconomic groups, as well as a broad range of natural aptitude.
Instead, I find that when schools are "doing better", there are commonalities like lower absence rates, lower suspensions and expulsions, lower rates of violence and legal problems outside of school, and higher score on polls targeted at measuring morale of both students as well as staff.
Students and teachers who are happy and less stressed will learn better, whatever that means for them. Whether that's achieving high marks on tests and report cards to move on to Harvard, or understanding better ways to grow potatoes and become a great farmer.
In line with this, I'd say that both better funding as well as proper allocation of that funding is badly needed in most school systems.
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