Reflecting the neglect of childhood disability in transitions research, there is a notable dearth of research on the factors shaping self-determination in post-school decision-making for disabled young adults. To address this gap, we explore how early educational experiences, parental expectations, economic vulnerability, school context, and educational supports shape perceived school support in developing self-determination skills among disabled and non-disabled secondary school students in Ireland. Utilising data from the nationally representative Growing Up in Ireland study, descriptive analyses map post-school decision-making and self-determination skills development among disabled and non-disabled young adults at age 20. Multivariate analyses explore the experiences of students with different disabilities, investigating how family, school, and peer influences shape such skill development. Disparities are found between disabled and non-disabled students, as well as among disabled students in school support for this skills development, with gender, socioeconomic background, cultural capital, and early educational experiences also important in perceived support. Positive school engagement and student-teacher relationships, as well as high self-expectations, emerge as protective factors, indicating that fostering supportive environments and self-concept may enhance students’ self-determination skills. The findings underscore the importance of promoting causal agency, providing support for proxy agency, and taking action to create enriching opportunities and choices for all students.