Well, in 1912, kids were learning things completely different than what we learn today, so of course you're not going to know any of it. Saying that people are getting dumber is a pretty ridiculous statement.
Agreed, there is only so much information we can plug into these kids during their school years. We simply have a lot of other things that are more important at this point. There rest can still be acquired on your own time...
"Dumbing down", when associated with education/schools has become acceptable slang for "less educated", so no, it's not "pretty ridiculous" at all.
Maybe they're just teaching them less important stuff these days. The criteria on that test shows some important information/skills to know.