It has been seven years since the central air conditioning system worked at the New York City middle school where Lisa Fitzgerald O’Connor teaches. As a new school year approaches amid the coronavirus pandemic, she and her colleagues are threatening not to return unless it’s repaired. Her classroom has a window air conditioning unit, but she fears the stagnant air will increase the chances that an infected student could spread the virus. “Window units just aren’t going to cut it. We don’t want to stay cool, we just want the air to flow properly,” said O’Connor, a science teacher who has worked at the Patria Mirabal School in Manhattan since 2009. “We are really super stressed out about it.” Schools around the country are facing similar problems as they plan or contemplate reopening this fall, dealing with aging air conditioning, heating and circulation systems that don’t work well or at all because maintenance and replacement were deferred due to tight budgets. Concerns about school infrastructure are adding momentum to plans in some districts, even in colder climates, to take classes outdoors for the sake of student and teacher health. Nationwide, an estimated 41% of school districts need to update or replace their heating, ventilation and cooling systems in at least half their schools, according to a federal report issued in June. There is no evidence that the disease can spread through ventilation systems from one classroom to the next, according to Dr. Edward Nardell, a Harvard Medical School professor who specializes in airborne diseases. The danger, Nardell said, is from ineffective systems that don’t remove floating viruses and let them linger in classrooms after they are expelled in an infected person’s breath, sneeze or cough.