School leaders at an urban high school in the U.S. Midwest encouraged teachers to use formative assessment to help students meet learning goals; however, several years later, they found inconsistent implementation. Without a clear understanding of teachers' formative assessment practices, leaders could not establish needed supports for its consistent use in the classrooms. The purpose of this bounded qualitative case study was to examine teachers' formative assessment use to check for student understanding and to adjust instruction. Black and Wiliam's formative assessment theory formed the foundation of this study. Research questions focused on teachers' perceptions of formative assessment and usage of formative assessment for instruction. Ten state certified high school teachers, who had at least a bachelor's degree, passed basic skills and subject area examinations, and taught within their majors or minors, were purposefully selected to provide data. Data were gathered from observations, interviews, and teacher logs and were analyzed inductively using open and axial coding strategies. Results showed teachers collected and used formative assessment to modify instruction and determine student understanding from a limited number of students. Furthermore, they lacked the knowledge, skills, and strategies to implement formative assessment to help all students meet learning goals. Based on the findings, 3 professional development (PD) sessions were created to help school leaders provide support for teachers' consistent formative assessment implementation. These endeavors may contribute to positive social change when administrators provide teachers with PD to increase teachers' knowledge and skills using formative assessment, and, ultimately, to meet student learning goals.