This report presents the results of a conducted by the authors in collaboration with the San Diego Unified School District (SDUSD), the second-largest district in California. For this study, the authors compiled a highly detailed, student-level database that enabled them to link factors influencing student achievement in ways that have not been possible with the state-level data generally used in such studies. In this report, they examine resource inequalities across schools, explore trends in achievement, and, most important, provide detailed statistical estimates of the school and classroom factors that most influence student achievement. Findings include: (1) The lowest socioeconomic status (SES) schools generally receive fewer resources than more-affluent schools, especially in the case of teacher qualifications in elementary schools; (2) An individual student's rate of learning is influenced by the academic ability of peers in his or her classroom and grade (classroom-level peer effects are stronger in elementary school while grade-level peer effects are stronger in middle and high school); (3) Class size influences gains in reading achievement in elementary grades but does not appear to be of significant importance in middle and high schools and (4) Teacher qualifications can make a difference, but the various measures of qualification have sporadic and varying effects in elementary, middle, and high schools, as well as on gains in math and reading achievement. The authors conclude the study with a discussion of the implications of their findings in light of the financial reality facing most school districts as a result of California's budget deficits. The following are appended: (1) Methods Used to Take Account of Unobserved Factors Affecting Student Learning; and (2) Details on the Regression Models for Elementary School Students. The remaining five appendixes are available via the Web only. (Contains 34 figures, 20 tables, and 40 footnotes.)