Researchers have identified self-determination as a key predictor supporting the transition process and achievement of positive in-school and post-school outcomes in adolescents with disabilities. Self-determination is also a malleable characteristic, as various factors influence its development and expression. However, the influence of this construct on student outcomes is not well-studied in autistic adolescents. In addition, while past studies have examined the effect of characteristics at the individual and family levels on self-determination, few studies have examined the impact of environmental predictors at the school level (Moran et al., 2021). To address these gaps in the literature, I examined the mediating role of self-determination on individual (intellectual functioning, adaptive behavior) and school factors (autism school program quality) and student in-school outcomes (academic achievement and IEP goal attainment). This study draws data from the Center on Secondary Education for Students with Autism Spectrum Disorder (CSESA; Hume et al., 2022; n=220), and I employed multilevel modeling techniques to account for the nested structure of students in schools. Results suggest that self-determination is a promising predictor of IEP goal attainment for autistic high school students. There was no notable clustering of self-determination at the school level, implying that school-level factors may not likely account for differences in students' level of self-determination. Mediation analyses suggest that self-determination may be a positive mediator between adaptive behavior and IEP goal attainment, especially transition goal attainment, although the mediation effect is likely small. More work is needed to elucidate the relationship between self-determination, specific goal areas, and academic achievement. Further investigation into the impact of school environment characteristics, such as classroom, on self-determination for students on the autism spectrum is also warranted. Implications for research and practice in schools for transition-aged autistic youth are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]