The number of United States public schools offering single-sex education for girls has increased dramatically in the past decade. Rationales for all-girls schools are diverse and grounded in differing gender ideologies. This article examines  reasoning about all-girls schools among school stakeholders (i.e., individuals affected by single-sex schools, including students, parents, and teachers) in the Southwestern United States. Specifically, they examined rationales for single-sex schooling related to gender differences in learning, gender differences in interests, girls’ ingroup preference, and gender discrimination. Endorsement of rationales differed across participant role (student, parent, teacher) and school type (single-sex, coeducational). Overall, stakeholders affiliated with an all-girls school were more supportive of each rationale than stakeholders affiliated with coeducational schools. Teachers affiliated with the single-sex school strongly endorsed gender differences in learning as a rationale for single-sex schooling. Endorsement of rationales did not vary across participant gender. The implications of these findings for educational policy and the interpretation of research on single-sex schooling are discussed.