How does student behavior respond to extreme temperatures? Using daily student-level data from a large urban school district, I estimate the causal effect of temperature on school absences and disciplinary referrals. Absenteeism increases in response to hot and cold conditions, particularly for Black, Hispanic, and lower-income students. Hot conditions also increase the likelihood that a student will receive a disciplinary referral, an effect found only among students attending schools without air conditioning. Results suggest that warming temperatures may lead to more student behavioral problems and that unequal access to air conditioning may exacerbate racial, ethnic, and socioeconomic disparities in school.