School psychologists pay a critical role in the assessment and intervention of students in special education. Within this role, they are highly likely to interact with students with intellectual and developmental disabilities (IDD), particularly in the context of school-based assessment associated with special education. Although students with IDD frequently interface with school psychologists during the evaluation process, they continue to face significantly poorer post-school outcomes and family school partnerships than their peers within and outside of special education. Following a content analysis and review of assessment-specific issues, population-specific needs, and previously proposed assessment models, the Collaborative-Adaptive Student-Centered (CASC) framework is proposed. This theoretical framework utilizes a content analysis of practice-oriented approaches that target assessment in order to improve the outcomes experienced by students with IDD and facilitate role expansion for school psychologists. Following a discussion of the CASC framework and associated practice tools, this paper reviews implications of a shift in assessment practices for students with IDD, their families, and school-based practitioners. Barriers to implementation of the CASC framework are also discussed.