The 5-month-old girl died was found Friday afternoon in the vehicle in the parking lot of Riverside High School in El Paso, Texas, police said.
The baby was taken to the hospital, where she was declared dead.
The baby's mother, Wakesha Ives, 37, is a business teacher at the school.
El Paso police spokesman Detective Mike Baranyay said he couldn't say how long the baby was left in the car, as the investigation is continuing.
Officials would also not comment on what may have lead to the baby being left in the car, the El Paso Times said.
No charges have been filed and the baby's name isn't being released.
Child-safety advocates said it only takes a few minutes for temperatures in a car to become dangerously hot for a young child. About 38 children die each year from heatstroke after being trapped in cars, they said.