Affective political polarization, or a sense of political identity-based animosity or distrust, became especially heightened during the Trump presidency. However, we know little about how youth experience such polarization in school and its effect on their political socialization. With unusual access to high-status independent schools, this paper explores the characteristics, consequences, and evolution of affective polarization for adolescents during high school. We draw on open-ended responses to school climate surveys administered at nine schools, 2018-2019, in which many students described experiences of political polarization and partisan hostility present in their schools. These reports of affective polarization increased by students' grade level. Further, we identify consequences of affective polarization on students' political socialization through school-based peer interactions such as threats to social status, lost relationships, and the inability to speak across political differences. These findings illustrate how partisan divisions in contemporary national politics may influence the political socialization of youth in schools and demonstrate the ways schools provide democratic education, separate from any political content in the curriculum, through the social interactional spaces they create.