Most big HVAC systems have this ability. On a rooftop unit usually it's as simple as a hatch that you slide; a split system would have to have a separate fresh

Most big HVAC systems have this ability. On a rooftop unit usually it's as simple as a hatch that you slide; a split system would have to have a separate fresh air duct you can control with a damper. Newer systems typically have explicit fresh air makeup systems with heat exchangers. Most schools with forced air ventilation should already have such a capability, if they bothered to look. If not, if would certainly be cheaper to have one installed than to leave the windows open. But of course, it may be easier to do one than the other bureaucratically.