Police say that they are becoming increasingly concerned about the safety of Lucy Allenby, from Ascot in Berkshire.
Her father Harry said: "Lucy, wherever you are please contact your family.
"We are all very worried and upset and need to know that you are alright."
Safety
Mr Allenby urged Lucy: "If there is anything the matter we can talk about things.
"Please just get into contact with us as soon as possible."
Mr Allenby said an application form for a scholarship to study modern languages had arrived on Saturday.
"We know you have had your heart set on this for a long time," he said.
"This is an opportunity you have looked forward to."
'Open mind'
Lucy was first reported missing by her parents early on Tuesday.
It appears the schoolgirl left the house on Monday night "of her own accord", a Thames Valley Police spokeswoman said.
"While police are keeping an open mind about the disappearance of the 15-year-old girl, we must emphasise that at this stage there is nothing to suggest that the incident should be treated as suspicious," she said.
"Police are continuing their investigations and would urge anyone at this time with information or sightings of Lucy or herself to come forward."
Lucy is 5ft 5ins tall, of slim build, with long blonde hair.
She was last seen wearing a denim jacket, a black top, black frayed boots, and a skirt.