High school achievement results have stagnated, even as elementary and middle school achievement has risen significantly since the 1990s. Student-level data from the National Assessment of Educational Progress (NAEP) strongly suggest that stagnant high school achievement is a real phenomenon. This result is consistent across different versions of NAEP and does not appear to result from changes in who is taking the test, flaws in test design and administration, or declining student effort. We recommend several improvements to NAEP, including the regular assessment of high school students in each state, and call for a renewed focus on high schools.