A study of 13 districts and nine states found that, “in some districts eighth-grade teachers, on average, are teaching content typically found at the end of fifth or the beginning of sixth grade internationally, while in other districts the content comes closer to that found at the end of sixth or the beginning of seventh grade…. In all 13 districts (and the 60 Michigan and Ohio districts), none taught content typical of the eighth grade internationally.”
The study calculated 8th-grade level as an average “across the nearly 50 countries participating in the 1995 TIMSS curriculum analysis.”
Source: William H. Schmidt, Leland S. Cogan, Richard T. Houang, Curtis C. McKnight , “Content Coverage Differences across Districts/States: A Persisting Challenge for U.S. Education Policy.” American Journal of Education, Vol. 117, No. 3 (May 2011), pp. 399-427.