Gender segregation exists in all walks of life. One of the most common forms of institutionalized gender segregation is perhaps single-sex schooling. Because schooling experience has important influence on students' psychosocial development, interest in gender-segregated education has been reviving over the globe. Skeptics of single-sex schooling have suggested that such schooling may increase students' gender salience (awareness of gender in categorizations), reduce opportunities for mixed-gender interactions, and increase mixed-gender anxiety, but little evidence has been found. It is critical to explore how single-sex schooling is associated with these psychosocial outcomes in adolescents and young adults because they are in the developmental stage when the desire and need to establish mixed-gender relationships increase. We report two systematic studies on gender salience, mixed-gender friendships, and mixed-gender anxiety on 2059 high school students and 456 college students from single-sex or coeducational schools. Even with demographic background controlled, results suggested higher gender salience in single-sex school students in the high school sample, and greater mixed-gender anxiety and fewer mixed-gender friendships in these students in both samples. These differences were not moderated by student gender and were similar in first-year versus senior college students. Moreover, mixed-gender friendships, though not gender salience, appeared to engage in a possibly bi-directional mediation relationship with mixed-gender anxiety that is consistent with a vicious cycle of escalating anxiety and lack of mixed-gender interaction among single-sex school students. These findings help fill the knowledge gap about the correlates of gender-segregated schooling and shed light on the precursors of later social and achievement differences between single-sex and coeducational school students.