Nice find.
If you research educational data in the first half of the 20th century in the United States, you will find it quite common that many children only completed their primary education. Parents during the Depression of the 1930's were forced to have their children enter the work force instead of going on to secondary education. This was especially true in rural areas.
In 1940, only 50% of students completed enough credits to receive a High School diploma. The majority of these graduates were female.
So this census record provides little out of the ordinary when compared to the average citizen of the day. In the 1920's and 30's, very few people attended college.
Of course the self proclaimed anointed who made up the Watchtower's leadership weren't just "average citizens" .... but that is another topic all together.
ginger