Since 2004, the staff of the Know Your Schools~for NY Kids project has been conducting studies to examine what marks the difference between high- and average-performing schools. No matter what the level of schooling, the two groups of schools differ in how they use evidence to make decisions. In general, high-performing schools' approaches to using data are more robust--data analysis and use are widespread among teachers and administrators, who collect a variety of data, collaborate around the collection and interpretation of data, and make decisions strategically on the basis of those data. This is true at all school levels, and building leaders play a key role in developing and fostering an inquiry-based approach that feeds a continuous improvement process. In addition to analyzing state assessments results and department-wide benchmark or quarterly exams, leaders in the high-performing schools also survey teachers, parents, and students about their experiences. Leaders of the high-performing schools also continually use data to monitor academic progress, and they let the data help them identify areas that need attention. Foundational differences between average- and high-performing schools across all the studies are the level of trust and the willingness to collaborate among teachers and between teachers and administrators. When trust is high and relationships are strong, teachers share data with colleagues in a joint and productive quest for improvement.