Most US students attend coeducational classes, but to what extent do students feel integrated into the entire classroom of their peers, especially with other-gender peers? The major goal of this research was to investigate how variations in gender integration (GI), measured by students' expectancies about inclusion, efficacy, and social costs of interacting with other-gender (OG) peers, predicted school liking and classroom supportiveness over an academic year, using a short-term longitudinal design. We also explored how students' expectancies changed over the year. Participants included elementary school students (515 school-age children; 51% boys, M[subscript age] = 9.08 years, SD = 1.00; 3-5th grade; 26 classrooms). A two-wave latent change score model showed that changes over the year varied depending on type of expectancy, grade, and gender, with decreases in inclusion and efficacy for boys. Longitudinal path analyses conducted to assess whether GI expectancies predicted school belongingness showed that students' levels of other-gender inclusion in the Fall uniquely predicted changes in levels of school liking and classroom community over the year, even with many controls in the model. The findings demonstrate that students' relationships with other-gender peers matter for having a sense of belonging in school, and educators should support and encourage these relationships. [This is the in press version of an article that will be published in "Psychology in the Schools."]