The purpose of this study was to examine the relationship between school climate and academic performance in two different ways: (1) by comparing the academic performance of different schools with different levels of school climate and (2) by examining how changes in a school’s climate were associated with changes in its students’ academic achievement. To examine how school climate and academic performance are related, this study analyzed grade 7 student data from 2004/05 to 2010/11 from the California Healthy Kids Survey, the California Standardized Testing and Reporting program, and the California Basic Educational Data System for 978 middle schools in California. School climate was measured by a set of student survey questions that assessed students’ perceptions about six domains of school climate. Schools with positive school climates were those in which students reported high levels of safety/connectedness, caring relationships with adults, and meaningful student participation, as well as low levels of substance use at school, bullying/discrimination, and student delinquency. Regression models were used to estimate the relationship between student-reported school climate and students’ average academic performance across schools. Regression models were also used to estimate how, for a given school, academic performance changes as school climate changes. All models included controls for racial/ethnic composition, percentage of English learners, and percentage of students eligible for free/reduced-price meals. The study found that (1) middle schools with higher levels of positive student-reported school climate exhibited higher levels of academic performance; (2) increases in a school’s level of positive student-reported school climate were associated with simultaneous increases in that school’s academic achievement; and (3) within-school increases in academic achievement associated with school climate increases were substantially smaller than the academic performance differences across schools with different school climate levels. As positive school climate is continuing to gain more attention as a lever to improve student learning, there is increasing interest in how improvements in school climate are related to improvements in academic performance. Most studies examining the school climate-academic performance relationship compare the academic achievement across schools with different levels of school climate. Although the results of this study found that schools with high levels of positive school climate exhibited substantially higher levels of academic performance than their counterparts with low levels of positive school climate, such differences across schools were not an accurate guide for predicting the magnitude of school-specific gains in academic performance associated with increases in school climate.