The huge police operation, including 100 specialist officers, was combing difficult terrain for any sign of her.
Claire Hart left home at 8.25am on Thursday for the two-mile walk to Dane Valley School along part of the A536 main road into Congleton, Cheshire, taking a short cut. The teenager was last seen in a field at 8.40am, chatting to a youth in combat gear with a shotgun slung over his shoulder.
Yesterday Claire's black Marks & Spencer coat with a fur collar was found near to where she was last seen.
Police said her chances were now "not looking good", although her parents were still hoping she would be found safe and well.
Superintendent Derek Barnett said both parents were "distraught" and her mother had visited the officers co-ordinating the search from the school.
He said his concerns were growing: "But, nevertheless, our officers are fully committed and are doing everything they can to find Claire."
A youth is being questioned by police in Congleton after magistrates granted a 36-hour extension to his custody from 9pm yesterday.
Sgt Chris Jones, of the Cheshire Police operational support unit, is leading the search from a base set up in the grounds of Claire's school.
The river Dane has been a focus of the search, but with its steep banks 50 local mountain rescue volunteers have been drafted in because of their expertise with ropes.
Another 50 specially trained police officers have been involved, including aircrew, frogmen and canoeists who are extending the search up to six miles downstream for more clues.
Residents in Eaton, Cheshire, described Claire Hart as a happy, lively girl who could be a little naive and trusting but took a full part in village life.
Near neighbour and Eaton parish councillor Roy Waltho said Claire was "a lovely, cheerful" girl. "She and her sister were adopted and arrived around four years ago. She has a good home there.
"We used to see her going to school, she always gave a shout and a wave and always made a fuss of our dog.
"She shows no fear and accepts people for what they are, she's not afraid of speaking to anybody. She's a rather naive, trusting girl."
He said the village was "in shock". "Another neighbour said she had only moved in recently, but Claire and her sister Michelle, 10, had made an effort to come over and introduce themselves.
"They were the sort of children it was a delight to have around."
The village of Eaton has about 275 inhabitants, mainly commuters, who have known little crime aside from an horrific incident that made headlines three years ago.
On the day before Christmas Eve 1994, former school dinner lady Tracey Mertens, 31, from Birmingham, was found dumped on the village's church steps suffering from severe burns.
She died in hospital soon afterwards. No one has been tried for her death.