The current study is a conceptual replication of the influence of an advanced educational opportunity and several student propensities to learn on a college-readiness assessment for mathematics (ACT scores) among an important and under-studied group of students. We focused on a sample of predominantly Black students from economically-disadvantaged homes in the U.S. (n = 329; 80% Black, 59% female, 66% whose parents had a high-school diploma or less). Accelerated course placement (high-school math course in 8th grade) was a unique predictor of math ACT scores above and beyond prior math achievement and other variables (B = 1.5). In line with a fade-out model, executive function skill in 6th grade was not a unique predictor with other predictors in the model. In line with emerging findings with marginalized students, mathematics self-concept in 6th grade was also not a unique predictor. Finally, students' mathematics achievement at age 4 predicted their math ACT scores in 12th grade (B = 0.69), as did their middle-school mathematics achievement in number, algebra, and geometry (B = 0.43-0.67), which mediated the relation between mathematics achievement at age 4 and math ACT scores. Findings highlight the importance of advanced academic opportunities, which national data indicates marginalized students are less likely to receive, the diminishing role of individual differences in executive function skill as children develop domain knowledge, the diminished role of math self-concept for predicting achievement among marginalized students, and the strong stability in math achievement.