This is an excellent post, and I think it makes an excellent point!
That being said, I want to say a little in it's defense. I don't think it looks THAT hard. True, I don't think I could successfully pass today. But I think if I studied these topics for a while, learned a list of the 9 parts of speech, the measurement of a bushel, history and geography, spelling rules, etc. then I could pass. In other words, if I'd just completed 7th and 8th grade.
My point being that this is a little bit like "Are You Smarter than a 5th Grader?" -- there are some things that you learn, and then forget later on. Just because an 8th grader could pass this test at that time doesn't mean that the same person could necessarily pass the same test 20 years later. I also don't know that I could pass our contemporary 8th grade exam, and I am a college graduate. I just haven't been studying these things for many years. You know what I mean?
This test does indicate that at 8th grade, children already had an excellent and comprehensive general understanding. I am impressed! Hey, at least they were LITERATE (as compared to graduating high school while being functionally illiterate).