Background/Context: Recently emerged with the implementation of the California's Public Schools Accountability Act of 1999 and the NCLB Act of 2001 is an increase in the number of education production function studies estimating the relationship between educational inputs and APIs. While the majority of past research on California school performance focuses on the impacts of different demographic measures and school resources on API scores at interschool level, few are done to study the effects of changes in similar factors on performance gain at intraschool level. Given that school performance is to be measured against oneself over time under California's current accountability system, the need is great to understand how school performance gain is affected by changes in student demographics and school characteristics within the school. Objective Of Study: The primary objective of this study is to investigate how APIs change with student demographics and school resources within individual schools. It is hypothesized that changes in factors contributing to interschool variations in API may also affect school API gains. In addition, the impacts of these variables on API gains of individual schools are then compared with results from prior cross-sectional studies to see if their effects on school performance differ between and within the schools. Research Design: Using the fixed effects regression a hypothetical causal relationship proposed between API gains and changes in nine student demographic variables, including seven racial/ethnic, free and reduced price meal and English language learning subgroups, plus seven school resource variables was estimated. Findings: School API gains appeared very sensitive to changes in all 16 variables. A 1% change in student demographics at school level was significant enough to change API by an average of -5.0077 to 1.2372 points, while changes in school resources by 1 unit was found to affect school API in the range of -0.0212 to 2.5013. Conclusions: While California places great responsibility on individual schools for student growth, little policy consideration is given to the likely effects of demographic and resource changes on school performance within the school. Moreover, this study's confirmation of the positive impact of teachers' advanced degree and full teaching credential on performance gains suggests that teacher qualifications may hold the key to improving student achievement.