How does temperature affect the human capital production process? Evidence from 4.5 million New York City high school exit exams indicates that heat exposure may affect educational performance in both the short and long run. Taking an exam on a 90˚F day relative to a 72˚F day results in a reduction in exam performance that is equivalent to a quarter of the Black-White achievement gap, and meaningfully affects longer-run educational outcomes as well, leading to a 12.3% higher likelihood of failing a subject exam and a 2.5% lower likelihood of on-time high school graduation. Furthermore, cumulative heat exposure over the course of the preceding school year may reduce the rate of learning as seen in exit exam scores, controlling for the short-run effect of exam day temperature. Teachers try to offset some of the impacts of exam day heat stress by selectively boosting grades for students who experience particularly hot exam sittings, perhaps in response to low levels of classroom air conditioning.