Below are the results from the Early Childhood Longitudinal Study Kindergarten Class of 1998-99 (ECLS-K) and Kindergarten Class of 2010-11 (ECLS-K:2011). Scores for general knowledge (for grades k and 1), science (for grades 3, 5 and 8), and both math and reading (for grades K, 1, 3, 5, and 8) were available. Pooled standard deviations for the groups being compared were used, except for ECLS-K:2011, for the reason noted. Excel file: here. To summarize the gaps, I averages Math and Reading scores in addition to averaging all scores. I also creased an average of all scores across all years.
Point of interest: the Black-White reading gap is down to one-third of a standard deviation at age 5 as of 2010-2011. (Note: It’s not obvious to me why the B/W gap increases, as it does, with age. The rate of the growth of the gap is too large to be accounted for by a simple genetic model (i.e., solely due to increased WGH with age).