In present analysis, an attempt is made to identify perceived social support mechanisms through which the gender composition of the school may influence pupils' stress responses. Use was made of data from 68 academically oriented secondary schools in Flanders, Belgium. Of these schools, 25 were mixed-sex schools and 43 were single-sex schools (21 girls' and 22 boys' schools). Respondents (3,370 girls and 3,057 boys) were 3rd-year students, ages 14 and 15. A multilevel analysis (Hierarchical Linear Modeling) was performed adjusting for parental socioeconomic status (SES), parental support, academic performance, curriculum enrollment, school mean SES, sense of belonging in school, and quality of teacher-pupil relationships. Results show that early adolescent girls in single-sex schools experience lower levels of stress than coeducational schoolgirls do and that this effect is largely accounted for by sense of belonging.