Boys and girls sit together in most classrooms, but do they interact? Based on 40-year-old evidence, coeducational classes may not be coeducational but instead segregated by gender, which may undermine student success. Our goal is to answer this question in today's classrooms. We used longitudinal data to assess gender segregation in 26 classes in 3 US coeducational elementary schools over an academic year. Third- to fifth-grade students (n = 515) from diverse backgrounds were asked how often they work with (frequency) and how well they work together (quality) with each classmate. Analyses illustrated a strikingly consistent pattern: for every grade, gender, and classroom, and across both fall and spring, students reported that they interacted more frequently with and had higher-quality interactions with same- than with other-gender classmates. Given the findings, teachers should encourage mixed-gender interactions; students likely will benefit socially and academically from these efforts.