The increased cost of the government schools, relative to the cost of everything else

Here's a graph showing the increased cost per pupil (since 1919) as compared to the increased cost of everything else.

Go here for more details.
One other thing, slightly off topic....The Federal Reserve was founded in 1913, and given the task of stabilizing the nation's money supply, which had been fairly stable until that point.  That's why most studies of runaway inflation begin around 1920, after the Fed had a few years to screw everything up. 
Say you're a prudent and thrifty ninety-year-old, and you saved $100 dollars under your mattress in 1920.  Relative to their original worth, those dollars will now purchase about $6.50 cents worth of stuff, by 1920's standards. 
We used to say that a dollar saved is a dollar earned.  WRONG !  A dollar saved is .94 lost. 

So here's a big "hope you go out of business" to the Fed, and to the public schools.