This dissertation examines current science education reforms by asking how it became historically possible to classify students, diagnose their differences, and prescribe distinct pedagogies in the name of inclusion. Through historicizing present pedagogical practices, the study explores how science education carries assumptions of human difference that undermine commitments to equality and justice. The study makes visible a paradox in pedagogies offered to empower marginalized groups by analyzing the ordering strategies that divide students as requiring distinct tiers of instruction. Juxtaposing historical analyses of research and policy with a two-year ethnographic study of high school teachers confronting racial disparities in tracked science courses, the dissertation traces shifting grids of classificatory practices in the 1920s and 2010s that direct teachers to see and sort difference in particular ways. Despite key shifts, tools to classify learners today retain norms that pathologize some people as less "scientific" than others. These tools make tracking appear like a natural response to pre-existing conditions (e.g., achievement gaps, differing interests, health disparities), obscuring how those distinctions were produced. The analysis underscores the danger in seeking to include underrepresented groups today without considering the ways in which these methods resemble strategies to improve the minds, attitudes, and hygiene of immigrant and colonized groups a century ago. The study rethinks the traditional conception that U.S. science education has only recently discovered diversity and has become progressively more inclusive since. An attention to the historicity of science pedagogy suggests that core debates within the field today--such as perceived tensions between scientific rigor and cultural relevance, or competing visions of preparing novice scientists versus future citizens--emerged in opposition to hopes and fears of Americanizing a racialized Other. Finally, the study raises implications for educational research, policy, and teacher education, particularly the importance of understanding how difference is ordered through the very techniques by which teachers are recognized as having professional knowledge of students and their "diverse needs." [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]