We examine how students' physiological stress differs between a regular school week and a high-stakes testing week, and we raise questions about how to interpret high-stakes test scores. A potential contributor to socioeconomic disparities in academic performance is the difference in the level of stress experienced by students outside of school. Chronic stress--due to neighborhood violence, poverty, or family instability--can affect how individuals' bodies respond to stressors in general, including the stress of standardized testing. This, in turn, can affect whether performance on standardized tests is a valid measure of students' actual ability. We collect data on students' stress responses using cortisol samples provided by low-income students in New Orleans. We measure how their cortisol patterns change during high-stakes testing weeks relative to baseline weeks. We find that high-stakes testing is related to cortisol responses, and those responses are related to test performance. Those who responded most strongly, with either increases or decreases in cortisol, scored 0.40 standard deviations lower than expected on the high-stakes exam.