I am not too excited by this graph to be honest. I took a look at some education and enrollment statistics from the Census. The base for the percentage is children over three enrolled in school. The analysis also includes college and graduate school enrollment. This explains why the Grand Forks and Cass counties show such a low number. Their university enrollments work against them in this regard.
I am not sure if it is a good thing that the numbers are so much the same across the different counties. A lack of variability here may imply a population structure that is also not very distinct across the counties.