This case study analyzes and documents factors that affect teacher learning and instructional practices in connection to the design your own (DYO) interim or periodic assessment process at one newly developed high school in New York City. Examining these factors through Riggan and Nabors Olah's (2011) conceptual framework offers insights into the complexity of using DYO periodic assessments to shift instructional practice and improve student achievement. Specifically, competing assessment purposes and teacher capacity in assessment development impacted assessment quality and resulting data and its use. Structures and systems for data reporting and use facilitated these processes. These insights can inform strategies for school leaders and policy makers seeking to support the use of DYO periodic assessments in school reform efforts.