Informed by complexity research and models for analyzing conditions in complex adaptive systems such as schools, I describe findings from a descriptive case study of influences on teacher decision-making about writing instruction in a high-stakes writing assessment grade. I highlight how the use of complexity as a theoretical framework for research provides a unique look at education systems, particularly looking at one teachers decisions across a school semester. I focus specifically on two conceptual models from the field of human systems dynamics (HSD), one used as a conceptual framework for complex adaptive systems, and the other used as a retrospective analysis tool in describing and explaining underlying conditions at work at a particular time for a particular decision.