In putatively meritocratic societies, doing well in school is a pivotal precondition for accessing further and higher education, which, in turn, has a pervasive, long-term influence on adulthood development. Yet, doing well in school may also predict "real-life success" outside formal education settings and independent of the educational qualifications that a person attains. Such predictions are likely to become salient during emerging adulthood, a life period characterized by career explorations and social-emotional adjustment. Here, we tested the predictive validity of end-of-compulsory school grades at age 16 years in a U.K.-representative population cohort sample of up to N = 6,488, who were born between 1994 and 1996, for a broad range of occupational, financial, and social-emotional outcomes at age 23. End-of-compulsory school performance accounted for 1%-20% of the variance across occupational, financial, and social-emotional outcomes in emerging adulthood. Educational attainment attenuated these associations only slightly, with school grades at age 16 accounting for variance in emerging adulthood outcomes independent of later educational attainment. We found that school grades were equally predictive for boys' and girls' outcomes. In children from lower family socioeconomic status (SES) backgrounds, school grades were more predictive of their educational attainment, financial attitudes, and anxiety compared to higher SES children, with varying effect sizes (i.e., 0.3%-4.2%). Our findings suggest that school-leaving grades facilitate the successful transition from adolescence to adulthood, independent of educational attainment, and that they might enable children from low-SES families to compensate for some of their background disadvantages.