The problem is not that we aren't buying enough education; it's that we're being sold a counterfeit.
http://nces.ed.gov/programs/coe/indicator_cmd.asp
We just blow it on administration-heavy bureaucracies and demonstrably failed methods.
Not to mention sports. The absurd amounts of money, attention and time poured into college athletics especially never ceases to amaze me. Even at the high school level, many students focus way too much on being "discovered" by talent scouts so that they can get scholarships to colleges they want - grades be damned.
Other posters have pointed out that there's plenty of spending in schools, but it is in frivolous activities on the periphery of education. They did not mention facilities, which I believe is another huge point of financial irresponsibility when it comes to how municipalities fund their schools. Any town pointing to a dilapidated school as proof of need for a new one is among the least likely to be a good steward of that school, given that it is deferred improvements and maintenance, not age, that really causes problems (to a point, of course). Worse yet, they move these schools to huge complexes on the edge of town so that no one can reach it without a car or bus. Then they get to spend more good school money on buses.
Money. There isn't enough of it. America would rather spend it on other "more important" things.