A police operation to trace Sara Jackson was last night intensifying, more than 24 hours after she disappeared.
Officers say the 16-year-old from Goldsmith Walk, in Lincoln, had been upset following the death of a teacher to whom she was very close.
Her father, Brett Jackson, said he was desperate for her to come home safely.
"If anyone sees her, please call police," he said.
"We miss her a lot and we want her back home.
"We just want to make sure she can send us a message that she is safe.
"We have not been getting a lot of sleep. We know it's cold outside, we know she did not have any money and we don't know where she is."
The teenager was last seen leaving Yarborough School in Lincoln at 3.10pm on Thursday.
Relatives said that she appeared to be in a good mood in the time before she went missing.
Concerns grew when she failed to come home and a police search began later that evening.
Sara has gone missing twice in recent weeks.
On both occasions she returned later the same day and officers are concerned that she has failed to do so this time.
Friends of the schoolgirl are now being questioned by detectives in the hunt for clues.
Sara is white, around five feet tall, with blonde, shoulder-length hair and blue eyes.
She was last seen wearing her school uniform with black boots and a black bag.
Anyone with information is urged to contact Lincolnshire Police on (01522) 532222.
Alternatively, ring Crimestoppers anonymously on 0800 555 111.