The "Self-Determination Inventory--Student Report" (SDI-SR) was developed to address the need in the field for new, theoretically aligned measures of self-determination. The purpose of this study was to establish the most robust and efficient set of items to assess the self-determination of adolescents with and without disabilities on the SDI-SR. Confirmatory factor analysis (CFA), using mean and covariance structures, was used to evaluate the factor structure of the SDI-SR to inform decisions on scale reduction. The items were tested across 20 groups generated by crossing disability (i.e., no disability, learning disability, intellectual disability, autism spectrum disorders, and other health impairment) and race/ethnicity (i.e., White, Black, Hispanic, and Other) groups. A robust set of 21 items that align closely with their associated constructs were identified. These 21 items showed strong measurement properties, including measurement invariance at the item level across the 20 groups. Implications for future research and practice are discussed.