Senior Sergeant Anthony Bond said police were called to a report of a car versus pedestrian in Shore St outside the school about 3.15pm.
The driver of the vehicle was 46-year-old woman and the pedestrian was 16-year-old girl, he said.
The girl was taken to hospital by ambulance with a possible head injury.
She was said to be in a serious condition by ambulance staff, Snr Sgt Bond said.