This study explored teacher perceptions of their ability to implement the Self-Determined Learning Model of Instruction (SDLMI) with fidelity and the impact of these perceptions on student self-determination outcomes on the Self-Determination Inventory: Student Report (SDI:SR). Using a piece-wise growth model, we found stability in teacher perceptions of their implementation over a 2-year period even with the introduction of a second transition-focused intervention to a subset of the sample. Using a mediation model, we then found a complex relationship between teacher perceptions of fidelity and student outcomes on the SDI:SR. We found that teacher-perceived fidelity over the course of the year was influenced by students' beginning of the year self-determination status and that teacher-perceived fidelity then affected students' end of the year self-determination status. The findings suggest an interactive relationship between teacher perceptions of implementation and student outcomes. Implications for research and practice are discussed.