Despite the prevalence of high-stakes assessments—and the growing likelihood of heat exposure during such assessments—the effect of temperature on performance has not yet been studied in such settings. Using student-level administrative data for the largest public school district in the United States, I provide the first estimates of temperature’s impact on highstakes exam performance and subsequent educational attainment. Hot temperature reduces performance by up to 13 percent of a standard deviation and leads to persistent impacts on high school graduation status, despite compensatory responses by teachers, who selectively upward manipulate grades after hotter exams.