Concerns about the breadth of the U.S. income distribution and limited intergenerational mobility have led to a focus on educational achievement gaps by socio-economic status (SES). Using intertemporally linked assessments from NAEP, TIMSS, and PISA, we trace the achievement of U.S. student cohorts born between 1954 and 2001. Achievement gaps between the top and bottom deciles and the top and bottom quartiles of the SES distribution have been large and remarkably constant for a near half century. These unwavering gaps have not been offset by overall improvements in achievement levels, which have risen at age 14 but remained unchanged at age 17 for the most recent quarter century. The long-term failure of major educational policies to alter SES gaps suggests a need to reconsider standard approaches to mitigating disparities.