Dumbing down education

I've recently paged through some McGuffey's Readers (my mother-in-law has her grandfather's). Dale has recently gotten on a Time-Life Books kick; we have the Great Ages of Man series and are looking to complete the Timeframe. They're basically the same, but the more recent ones have more white space, more pictures, and simpler text.
I think those two kind of bring into sharper relief the dumbing down of America. Yes, we can all find exceptions (I know someone who had Russian in high school), but as a general rule, the curriculum has gotten watered down. We might not notice it ten years later, but when you compare a century ago, it really shows.
Forty or fifty years ago, a guy could drop out of high school, walk to a factory, and get a job that would support him and his family until retirement. Why was this? Those factory jobs didn't take skills or they could be learned on-the-job. Why can't that happen today? Sure, automation has done away with most of those jobs, but I'd wager at least some of it is due to the average high school education isn't what it was.
Why? We hear that the information available doubles every six months, but does that mean we should learn all of it? The names of Gwyneth Paltrow's kids is information, as is Britney Spears' husband and Dan Brown's latest magnum opus. But are any of those things worthwhile? Do they make some sort of statement about the universal human condition? Will anyone care in a decade, let alone a century? Are they even questions in Trivial Pursuit?
But along with why, I'd like to know when it started. That would be some neat kind of sociological research, wouldn't it? What percentage of kids were in high school? What percentage went to college? What texts were studied a century ago? When did they get replaced, and by what? What were the reading levels of those texts as compared to the current ones?
And if kids are spending more time in school (more days and even minutes mandated by the state) but less time learning such things, what are they doing with that time?