Although poor air quality in a school can have multiple causes, the heating, ventilating, and air-conditioning (HVAC) system plays a major role. Suggestions that architects, facilities managers, school board members, and administrators can use in selecting HVAC systems are discussed. Focus is on the performance criteria for classroom systems, and includes temperature and humidity ranges, indoor air quality, energy use, operation and maintenance, simplicity, staffing and standardization, reliability, flexibility, vandal proofing, and capital cost. Detailed descriptions of the kinds of systems used for classrooms are also discussed, such as the use of unit ventilators, variable air volume systems, single zone systems, multizone units, water source heat pumps, and separate ventilation air systems. The advantages of each of these systems are presented. Other considerations facing decision makers include choosing between mechanical or natural ventilation, and figuring ways in which building design, climate, and capital costs will affect such decisions. Likewise, school officials must consider the specifics of location, architecture, use, and management of the facility. To achieve the performance objectives of any HVAC system, requires a balancing of air and water flows, testing performance, documentation, and using trained operating personnel. (Contains five illustrations and nine references.) (RJM)