Class size reduction (CSR) has emerged as a very popular, if not highly controversial, policy approach for reducing the achievement gap. This article reports on findings from an implementation study of class size reduction policy in Wisconsin entitled the Student Achievement Guarantee in Education (SAGE). Drawing on case studies of nine schools, we identify school principals as critical and overlooked influences in the implementation of class size reduction policies. Principals' influence proved central in three challenge areas: the use of space, serving the needs of diverse learners and building teacher capacity. By comparing the patterns of practice across schools, we have identified school leadership practices and dispositions for CSR that appear related to improving achievement levels. (Contains 2 tables.)