The number of students opting out of standardized tests has grown in recent years. This phenomenon poses a potential threat to our ability to accurately measure student achievement in schools and districts. This brief documents the extent to which opting out is observed in the CORE districts and models how higher opt-out levels could affect various accountability measures. More students opting out could significantly impact some accountability measures in use in California, but the CORE districts' growth measure is largely unaffected, as it reports the impact of schools on individual students' achievement. 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. This brief suggests that districts should consider explicitly adjusting for the characteristics of the students who actually sit for tests when designing school accountability systems.