Fully articulating validation arguments in the context of classroom assessment requires connecting evidence from multiple sources and addressing multiple types of validity in a coherent chain of reasoning. This type of validation argument is particularly complex for assessments that function in close proximity to instruction, address the fine granularity of learning trajectories (LTs), have multiple stakeholders, and are delivered digitally with a quick turn-around for formative assessment purposes. This article describes a validation framework for classroom assessment and uses it to illustrate a validation argument addressing one of several purposes for the assessments, the use of class-level data by individual teachers. The argument concerns the use of a middle-grades digital learning system, Math-Mapper 6-8, which contains LT-based diagnostic assessments. The argument is structured as a set of six claims that examine the assessment structure, the identification and treatment of non-conforming items, the analysis of student data, and the analysis of teachers' interpretations of data. The article stresses the critical role of scrutiny and debate among learning scientists, psychometricians, and practitioners in the validation process.