Sussex Police said that officers were concerned for the welfare of missing 16-year-old Lacey Easen who may be in Worthing or Brighton.
She failed to arrive at school in Worthing, where she has been living, on Tuesday (13 October) and has not been seen since.
She comes from Brighton and has many friends there.
Lacey was described as white, 5ft 3in to 5ft 4in tall, of medium build and with shoulder-length brown hair.
She was last seen wearing a camel below-knee-length coat with a fur-lined hood, dark leggings and black trainers and she was carrying a black handbag.
Police are concerned that Lacey has now been missing for more than three days.
Anyone who sees Lacey or who knows of her whereabouts is asked to phone Sussex Police on 101 or email firstname.lastname@example.org.