This report describes the adverse public-health effects resulting from releases of hazardous substances in schools. Data were analyzed from emergency events reported to the Hazardous Substances Emergency Events Surveillance (HSEES) system by 14 participating states during 1993-1998. Compared with all other types of events, a higher proportion of school-related events resulted in victims (relative risk [RR] = 3.94, 95 percent confidence interval [CI] = 3.37-4.60) and in evacuation (RR = 5.76, 95 percent CI = 5.16-6.43). The most common cause of these events was operator error, followed in frequency by equipment failure, improper mixing, and deliberate releases. The majority of victims were exposed to spills emitting noxious gases, and their resulting symptoms were primarily associated with the respiratory tract.