High schools are increasingly expected to use data for improvement and to provide evidence that programs and instructional practices are preparing all students to develop essential knowledge and skills. This publication describes how schools can develop the capacity to analyze and use data as a core component of improving secondary schools using the Breaking Ranks Model. Seven chapters are as follows: (1) "The Challenges of High School Reform" discusses the expectations for a 21st century high school; (2) "Putting Student Results at the Center of High School Reform" looks at student-centered accountability; (3) "Developing Capacity for Data-Driven High School Reform" considers why high schools resist using data-driven reform; (4) "The Breaking Ranks Model of High School Reform" describes the model's framework and key components; (5) "Data-Driven Reform in Low-Performing High Schools" describes how reform facilitators used data effectively in their work; (6) "The Schools' Experiences in Using Data" gives examples of how high school staff performed data-driven reform; and (7) "Making the Transition to Data-Driven High School Reform: Lessons from Research and Practice" identifies meaningful questions about student performance. Properly used, data can make a difference in meeting the needs of every high school student. (Contains 37 references and 8 figures.) (RT)