Across the nation, most states are implementing a new set of standards- and accountability-based reforms: the Common Core State Standards and their accompanying assessments. Unfortunately, the perspectives of school-based educators are largely missing from policy and implementation decisions about the Common Core. To address some of the gaps in previous research, the purpose of this dissertation—a comparative case study of two middle schools on the East Coast of the United States—was to describe and analyze school-based educators’ perceptions of and responses to the Common Core and the Smarter Balanced Assessment Consortium (SBAC) Exam. Data analysis revealed that educators in the two schools generally worked from an inquiry stance on teaching, learning, and schooling (Cochran-Smith & Lytle, 2009) in that they collectively and critically analyzed the intentions of educational policy and practice based in part on their beliefs about student-centered, constructivist teaching and learning. Consistent with this perspective, they made sense of the Common Core and SBAC based on the degree of alignment they perceived between their own educational values and beliefs, on the one hand, and the values and beliefs that animated the policies, on the other hand, which I conceptualized as “principled sensemaking.” How the educators actually implemented the Common Core and SBAC was the result of the intersection of their principled sensemaking of these standards-based reforms and the degree of agency they had over policy implementation. I termed this type of response to policy “principled implementation.” Four types of principled implementation were identified: principled adoption, principled neglect, principled compliance, and principled resistance. New understandings of school-based educators’ unique, critical, and nuanced perceptions of the Common Core and SBAC and how they believe the Common Core and SBAC influence teaching and learning have the capacity to inform decisions about the future of the Common Core in schools, and contributes to a broader understanding of how school-based educators take up and respond to standards- and accountability-based reforms.