If you graph actual NAEP since 1998 versus a 1998 group-fixed score, you'll see increases (1998-2002, 2005-2013, 2015-2017) and multiple declines twice taking the line back down to the 1998 linear trend prediction (2002-2005, 2017-2024).

It is wrong to get overly exact here though. We are talking about a overall score variance of just 10pts in the face of an achievement gap of nearly 30pts, with an absolutely massive 20% decline in white population share over the 26 year period.