The purpose of this study is to clarify the relationship between organizational citizenship behavior, principal support, and student achievement. It proposes that student achievement increases as teachers contribute beyond what is expected of them, and that they are driven to do this when they are emotionally supported by their principal. The study aims to expand the definition of instructional leadership to include expressive support and relationship-building. The paper employed multiple regression analysis of survey data from 1,104 teachers in 34 high schools in Virginia, USA, to examine the relationships between types of principal support and other school-level variables (i.e., OCB, achievement, SES). The analysis revealed a strong, significant relationship between expressive principal support and OCB, and between OCB and student achievement. The findings suggest that, compared to maintaining respectful professional relationships, instrumental support, and professional development are less important in eliciting extra-role behaviors from teachers. The paper includes implications for the preparation of principals, for the development of instructional leadership standards, and for reform policies that focus on short-term gains over more distal school climate improvements. This paper expands current conceptualizations of instructional supervision and contributes to a more collegial, less managerial, understanding of leadership for school improvement.