This study used qualitative interviewing with teachers at three high schools to answer research questions about teachers' assessment knowledge, school-specific assessment cultures, and teachers' perceptions of the assessment literacy of other key stakeholders. Data analysis revealed shared knowledge and practices across schools--use of formative assessment to guide instructional decision making; reliance on collaborative processes as the basis for cultivating school-wide assessment practices; and the belief among teachers that students, parents, and school administrators often hold naive views about the relative primacy of summative over formative assessment. Data analysis also identified distinctive features of the assessment culture at each of the schools. (Contains 6 footnotes.)