Shanice Buckle, 15, has not been seen for nine days since she kissed her dad on the cheek last Tuesday and left for school with only a small bag of possessions.
She never arrived, having called staff to claim she had a doctor’s appointment.
Her worried father Eric Buckle, 44, of Crosland Moor, Huddersfield, said: “The worst thing is the not knowing.
“She’s the first thing I think of in the morning and the last thing at night.
“Every time I hear the phone ring I wonder if it is going to be bad news.
“It’s a nightmare.”
Police said Shanice was last seen in Huddersfield town centre with two male friends and it is believed she travelled to London with the two friends and could now be in the Hackney area.
Shanice is believed to be with a 17-year-old boy with whom she had developed a relationship after they met through social networking site Facebook.
It is thought they may be with another older man.
Specialist officers have travelled to Swindon and London, where the 17-year-old boy, who is known to police, has connections.
It is thought Shanice may have sold her Blackberry mobile phone and a laptop to get cash. The last communication she had with her family was last Wednesday, the day after she disappeared, when she sent a brief text message to her father after he told her he was going to call police.
Mr Buckle said her disappearance was out of character and he feared the worst.
He said: “My daughter is caught up in something she has no idea how to handle and I believe she might not be able to walk away from it, even if she wants to.
“Every second that goes by I get more concerned. In the last seven or eight days I have had worst-case scenarios going through my head.”
In a direct appeal, he said: “You might think that I am angry but I’m not. Please send us a text message or give us a call just to let us know you are OK.
“Your family and friends are all worried about you. We just want you home safe.”
Shanice’s family have put posters around Huddersfield appealing for information and have set up groups on Facebook to spread the word.
Her sister Emma, 20, said: “You can’t help but think bad things. There’s no way of knowing what is happening.”
Headteacher Jane Acklam of Moor End Technology College in Huddersfield, where Shanice is a pupil and had been part of the Gifted and Talented programme, said: “She’s a very bright and articulate young lady who has a lot of friends.
“She has a superb attendance record and we are all missing her. We just want her to return quickly and safely.”
West Yorkshire Police said they believe Shanice could be in either Wiltshire or Hackney.
She is described as being of mixed race, about 5ft 5in tall with long, red and brown hair in ringlets.
She has a pierced top lip and was wearing a black cardigan, black blouse, a red and black tie and black leggings.