In 1987, parents and teachers from diverse neighborhoods of a local school district in New York (New York) founded the Bronx New School, a small public elementary school of choice that was meant to be learner-centered, with high standards for all. The school was organized into heterogeneous, multi-age classes and structured to encourage collaboration among faculty, students, and families. In spite of political stresses, the school's founding values have survived. This report focuses on the first 3 years of its life, a time when a comprehensive assessment system was designed and used throughout the school. The assessment system was designed to support instruction and learning through the collection of descriptive records of student growth. Teacher-kept records, student-kept records, and samples of student work in portfolios are used in combination to develop a picture of student learning. A developmental framework constructed by teachers provides a general guide for expectations of progress. The implementation of the system is illustrated through the case study of one student. (Contains 31 references.) (SLD)