The number of students opting out of standardized tests has grown in recent years. This phenomenon represents a potential threat to our ability to accurately measure student achievement for schools and districts. This working paper documents the extent to which opting out is observed in the CORE districts. It then models the extent to which various accountability measures would be impacted by growth in the rate of opting out. The growth of opting out could significantly impact some accountability measures in use in California, but the CORE growth measure is largely unaffected. In contrast, accountability metrics that track student achievement by cohort are at risk of becoming biased even with relatively low absolute levels of opting out, and districts should consider explicitly adjusting for the characteristics of the students that actually sit for tests when designing school accountability systems.