Adolescent voluntary associations are particularly well positioned in the life course to encourage voting as youth become full citizens. Extracurriculars socialize students into voting by habituating them to civic engagement and by connecting them to politically engaged cultures. We establish this argument by testing the effects of high school extracurriculars on voting and the formation of political ideology in young adulthood, using two nationally representative longitudinal datasets and propensity score matching. We find that participation in general promotes voting, though some activities (notably, some sports) decrease it. Specific activities that encourage voting often have no political content, and their effects are not explained by the voting rates of peers in these groups. One of the biggest and most robust effects is for the performing arts: participation in high school performing arts is related to a higher rate of voting in early adulthood. Furthermore, some activities affect political ideology and party membership in adulthood, illustrating socialization into distinct political cultures. The overall pattern is that religious attendance and a few sports steer students to the conservative end of the political spectrum and into the Republican party, while academic clubs, drama clubs, and honor society steer students towards the liberal end and/or into the Democratic party. Schools can create environments that encourage extracurricular involvement through funding and policy. But they can also discourage extracurriculars through neglect. These results demonstrate that which activities thrive and which shrink will have an impact on future voting behaviors of young adults. (Contains 7 tables.)