It's not representative of the average American during that era - who would not have made it to 8th grade.
Heck, in 1910, Kentucky had a 17% illiteracy rate [0]
The fact that illiteracy using the IMF definition is non-existent in the US automatically means your average 8th grader today is better off than one a century ago.
And if I'm honest, the exam itself doesn't seem that different from content I dealt with in 4th-6th grade and I was attending an urban lower middle class school back then.
[0] - https://usa.ipums.org/usa/resources/voliii/pubdocs/1910/Vol1...