This study investigated how the school experiences and personal characteristics of youth with Intellectual Disabilities (ID) contribute to their longitudinal trajectories of anxiety. To this end, we relied on a sample of 390 youth with mild (48.2%) to moderate (51.8%) levels of ID, aged from 11 to 22 (M = 15.70), and recruited in Canada (n = 140) and Australia (n = 250). Across three yearly time points, all participants completed self-report measures of anxiety, school climate, and victimization. Our results revealed a slight normative decrease in anxiety over time and showed that experiences of school victimization were associated with higher levels of anxiety (initially and momentarily) and increases in victimization were accompanied by increases in anxiety over time. Perceptions of attending a school that fosters security and promotes learning also tended to be accompanied by lower levels of anxiety (initially and momentarily). Momentary increases in perceptions of attending a school that fosters positive peer interactions were associated with momentary decreases in anxiety, whereas momentary increases in perceptions of attending a school characterized by positive teacher-student relationships and an equitable treatment of all students both led to small momentary increases in anxiety once all other components of student school experiences were considered. The theoretical and practical implications of these results are discussed.