The accountability movement and high-stakes testing fail to attend to ongoing instructional improvements based on the regular assessment of student skills and teacher practices. Summative achievement data used for high-stakes accountability decisions are collected too late in the school year to inform instruction. This is especially problematic for students who require early intervention to remediate skill-specific difficulties, such as those identified for special education. The purpose of this chapter is to describe the School System Improvement Project's hybrid approach to utilizing both formative and summative assessments to (a) inform decisions about effective instruction based on all students' and teachers' needs, and (b) guide high-stakes decisions about teacher effectiveness. Five key components of the SSI Project are outlined, including: (a) the use of formative assessments; (b) data collection from multiple teacher and student measures; (c) an emphasis on instruction and service delivery for all students, including those with disabilities, from minority groups, or from marginalized populations, based on a continuum of need; (d) ongoing teacher and administrator support via a systematic problem-solving process and coaching; and (e) consideration of student growth or progress. The practical implications of this approach are provided along with recommendations for advancing research and policy.