Cross-national research finds that "shadow education" — educational activities outside of formal schooling — tends to confer advantages on already privileged students. Shadow education in the United States, such as test prep for college entrance exams, has received considerably less attention. Drawing on the National Education Longitudinal Study, we analyze the likelihood of participation in, and the implications of, SAT preparation. Social class inequalities in test preparation, particularly costly SAT courses and private tutoring, are notable and have at least moderate consequences for SAT scores and selective college enrollment. We also find racial/ethnic variations in the use of test preparation. We consider the implications of these findings for understanding shadow education, stratification and educational mobility in the United States.