NAEP doesn't report grade levels. It reports achievement levels, and these do not correspond to grade levels. Nor does he understand the NAEP achievement levels or just how demanding NAEP's "proficiency" level really is. To score below "proficient" on NAEP does NOT mean "below grade level."
NAEP has four achievement levels.
The top level is called "advanced," which represents the very highest level of student performance. Students who are "advanced" probably are at an A+; if they were taking an SAT, they would likely score somewhere akin to 750-800. These are the students who are likely to qualify for admission to our most selective universities.
Then comes "proficient," which represents solid academic performance, equivalent to an A or a very strong B. Guggenheim assumes that any student who is below "proficient" cannot read at "grade level." He is wrong.
The third level is "basic." These are students who have achieved partial mastery of the knowledge and skills necessary to be proficient. This would be equivalent, I believe, to a grade of C. Many (if not most) states use NAEP's "basic" as their own definition of "proficient." This is because they know that it is unrealistic to expect all students to be "A" students.
"Below basic" is the category that Guggenheim confused with "below grade level." In 8th grade reading, 25 percent of students are below basic, not 70 percent. If Guggenheim knew what he was talking about, he might have said that 70 percent of 8th grade students were unable to score the equivalent of an A, but that would not be an alarming figure. It would not be a very dramatic story had he said, in sonorous tones, "25 percent of our 8th grade students are 'below basic' in reading, and that figure includes students who are learning English and students with disabilities."