A growing number of educators concur that, in order to improve student academic performance, schools need to focus not only on students' academic needs but also on their social, emotional, and material needs (Piscatelli & Lee, 2011). As a result, school climate--the social, emotional, and physical characteristics of a school community (Cohen, McCabe, Michelli, & Pickeral, 2009)--is gaining more attention as a lever to improve student academic performance. Most studies on the relationship between school climate and academic performance assert that a more positive school climate promotes higher academic performance. But evidence of a relationship between the two is weak. These studies generally are based on data collected at a single point in time and compare academic performance across schools with different school climates. They show that academic performance is higher in schools with a more positive school climate at single points in time. However, little evidence exists that changes in school climate over time are associated with changes in academic performance. This study used grade 7 student data from the California Healthy Kids Survey and administrative data for approximately 1,000 middle schools in California for 2004/05-2010/11 to measure students' perceptions about six domains of school climate. Schools with a positive school climate were those in which students reported high levels of safety/connectedness, caring relationships with adults, and meaningful student participation and low rates of substance use at school, bullying/discrimination, and student delinquency. School-level academic performance was measured using grade 7 California Standards Test scores in English language arts and math. The study team examined the relationship between school climate and academic performance across schools to determine whether in a given year California middle schools with a more positive school climate had higher academic performance. The study team also sought to determine how academic performance for a given school improved as school climate improved by examining how changes in school climate over two-year intervals were related to changes in average academic performance. Key findings include: (1) Schools with a more positive student-reported school climate had higher academic performance in English language arts and math; (2) Changes in a school's student-reported school climate over time were associated with changes in academic performance at that school; and (3) The changes in academic performance within a school that were associated with changes in student-reported school climate over time were substantially smaller than the differences in academic performance across schools with different school climate values in a given year. For example, in a given year schools at the 50th percentile on school climate were at the 48th percentile on math performance, on average, while schools at the 60th percentile on school climate were at the 51st percentile on math performance. This finding suggests that an improvement of 10 percentile points in school climate would be associated with an average 3 percentile point increase in academic performance. However, when followed over time, schools with a 10 percentile point increase in student perceptions of school climate averaged a less than 1 percentile point increase in academic performance. The following are appended: (1) School climate domains measured on the California Healthy Kids Survey, grade 7 students; (2) Data and methodology; and (3) Cross-sectional and longitudinal associations between school climate and academic performance in percentile point and standard deviation metrics.