I would attribute it to cars and mass media, TV.
Still doesn't change that the high schools are doing a poor job in basics such as math, grammar, science, literacy, geography and history. In fact, if it is as I say -- that native IQ's are higher, then our high schoolers should be doing much much better than 1940's, and it is even more of a hit on the education system that they are not.
Maybe you should watch a couple of Jerry Springer or Maury Povich shows --- I'm not sure the IQ of the general population has gone up.