Drexel Preparatory Academy principal Cheryl Bowman told WTVF-TV students began coming to the office Monday morning complaining of flu-like symptoms. Officials decided to cancel classes.
After students were sent home, the school got a call from Vanderbilt University Children's Hospital, saying several students had been brought in and showed high levels of carbon monoxide.
Hospital spokesman Craig Boerner said Tuesday that 49 students were eventually brought in by parents and 40 of them were admitted for 23 hours of oxygen treatment.
Workers found a hole in a heating unit that serves the central part of the school. Carbon monoxide is a colorless, odorless gas produced by combustion.