Background: School-level socioeconomic status (SES) influences on adolescents' lifestyle behaviors is understudied. We examined how school-level SES and sex influence adolescents' health-related lifestyle behaviors and intentions. Methods: Grade 8 students aged 13-14 years completed an online questionnaire regarding their sociodemographic characteristics, dietary behaviors, physical activity participation and recreational screen-time, and intentions regarding these behaviors. School-level SES, based on an Index of Community Socio-Educational Advantage (ICSEA), was categorized as low or high. Generalized estimating equations estimated individual-level summary statistics, adjusted for clustering. Results: Students (N = 2538; response rate = 79%) from 23 high schools (low ICSEA = 16) participated. Compared with low ICSEA students, high ICSEA students were more likely to report eating breakfast daily (OR 1.9 [95% CI 1.5, 2.4]), not drinking sugar-sweetened beverages (SSBs) daily (2.9 [1.9, 4.3]), and were more likely to have intentions to eat breakfast (1.8 [1.3, 2.3]) and = 5 vegetable serves (1.2 [1.0, 1.5]) daily. Boys were more likely than girls to meet recommendations for breakfast eating, vegetable intake, moderate-to-vigorous physical activity and screen-time, but boys were less likely to meet recommendations regarding SSB intake. Conclusions: Students from low ICSEA schools would benefit from additional support to improve dietary-related behaviors and intentions. More research is required to identify what targeted approaches will address sex differences in adolescents' lifestyle behaviors.