It is teaching necessary for salvation that more money is both necessary and sufficient to improve America's schools, and as is always the case with teachings necessary for salvation, you must accept this one completely on faith. Nonetheless, additional evidence has come to light that the "underfunding" manifests itself in every way except funding statistics.
I have previously noted America's rank among the OECD countries in education spending. Now comes a study from the Center for American Progress, which can hardly be dismissed as a bunch of reactionary wingnuts, on return on educational investment, including comparisons of school systems within each state covered. The plots of achievement against adjusted per-pupil spending show nothing like the strong correlation that everyone is sure exists.