This paper examined whether year round schooling (YRS) in California had an effect upon the outcome and growth of schools' Academic Performance Index (API) scores. While many previous studies had examined the connection between YRS and academic achievement, most had lacked the statistical rigour required to provide reliable interpretations. As a response, this study used data collected from 4,569 schools over six years and two integrated and more sophisticated statistical techniques--mixed analysis of covariance and latent growth model. Results showed that YRS did not affect either the outcome or the growth of API scores. (Contains 5 tables, 2 footnotes, and 4 figures.)