How do we define a high-quality school librarian? Decades of educational researchers have attempted to link teacher characteristics--such as how teachers are prepared, which credentials they carry, and years of experience--to student outcomes. These researchers have contended that individual educator attributes may have a direct effect on what and how much their students learn. School librarians are also teachers who have direct student contact, and although numerous studies have indicated that school librarian preparation, licensure, and other background characteristics are promising areas for further direct exploration, researchers have yet to examine if, how, and why school librarians' certification or preparation positively impacts students' learning outcomes. The purpose of this paper is to compare findings from causal educational research to findings from descriptive school librarianship research to discern possible areas of causal alignment that warrant further investigation. In this study, we present a subset of a larger mixed research synthesis of causal educational research related to student achievement, contextualized with existing school librarianship research, to draw relationships between classroom teacher and school librarian preparation and characteristics and to shape researchable conjectures about school librarians' effects on learner outcomes.