Here is a graph of the spending on education in public K to 12 schools in the US compared to the spending on education outside of K to 12 schools from 1950 to 2007. The data is from this site thanks to @jasonflom. Please comment below if you see an error in my math.

Graph of k - 12 vs non-k - 12 education spending

So what this graph shows is that there is an incredibly strong relationship between the money spent from all sources on public K to 12 education in the United States and the money spent from all sources on all other education in the US. Since the funding for sources outside of public K to 12 education have very different funding sources we know that this funding situation is not a function of federal or state policy.

In other words, since funding for education has been increasing regardless of the policy behind the funding, it must be that the education just costs more today than it did years ago. Some obvious reasons for this are that the technology in schools has changed over the years and costs more and land costs have risen above the cost of inflation.