Despite increasing awareness about the role of classroom assessments in perpetuating educational inequities, the research community continues to struggle with how to support teachers to design and use classroom assessments for achieving equity. In response to recent calls to better connect learning theory to the design of classroom assessments, we explore the links among contemporary learning theories, classroom assessments, equity, and teachers' professional learning. Building a conceptual argument that we should shift our attention from assessment tasks to a classroom activity system to better support minoritized students' learning via classroom assessment, we examine how teachers participate in assessment codesign activities in two research-practice partnerships (RPPs), and then identify emerging tensions in relation to promoting equity. Each RPP drew upon contemporary learning theories--sociocognitive and sociocultural learning theories, respectively--to create a coherent system of curriculum, instruction, and assessment. The examples show that the tensions emerging from each project are at least partially related to the learning theory that led the researchers to set up professional learning settings in a particular way. Our findings suggest that managing these tensions is an inherent part of the work as researchers seek to support equitable student learning. We discuss specific implications for the assessment community.