I went through and calculated the standardized Black-White difference based on the 2011-2012 NAEP 4th and 8th grade Math and Reading results. As testing for 12th grade is being conducted this spring, those 2011-2012 results are not yet available. The difference stands at approximately 0.9 SD. As the correlation between these tests and g is 0.7 (Gottfredson, 2005), and as the g differences at these ages is under 1 sigma, other factors in addition to g are implicated in origin of these differentials.
Gottfredson, L. S. (2005). Implications of cognitive differences for schooling within diverse societies.