As one of the stated goals of Common Core adopting states is to improve student achievement by holding students to more rigorous standards, the Brookings Institution has been tracking NAEP scores against CCSS adoption and strength of implementation. While fourth and eighth grade students from CCSS states performed better than their peers on the NAEPs between 2009 and 2013, students in non-adoption states made larger gains from 2013-15. This could signal that the Common Core effect is declining, although the report's authors also note that "nonadopters performed better than CCSS states, but only by declining less, not through improved performance. "
There are factors outside of Common Core, however, that can also impact student scores. The researchers are hopeful that future data sets will contain richer data that will allow for a more sophisticated analysis. However, the authors conclude that the results also support the hypothesis that the "CCSS has already had its best years and additional gains will be difficult to attain."
Read The Brown Center Report on American Education (March 2016).