Jamie Reynolds, who previously denied the charge, pleaded guilty to the murder of Georgia Williams during a hearing at Stafford Crown Court.
The 23-year-old was at the centre of a UK-wide manhunt in May after killing Georgia in their home town of Wellington, Shropshire, and dumping her body in woodland near Wrexham.
Reynolds, wearing a dark suit, a striped shirt and a purple tie, sat with his head bowed in the dock after entering his plea on what was scheduled to the first day of a three-week trial.
The shop worker, of Avondale Road, Wellington, was arrested at a hotel in Glasgow on May 29 this year, three days after his victim was reported missing.
Georgia, a member of the Air Training Corps who planned to join the RAF as a paramedic, knew Reynolds and is believed to have been killed at an address in Wellington.
The teenager's body was found around 50 miles miles away, off the Nant-y-Garth pass.
Reynolds, who will be sentenced later this month, is known to have killed Georgia after inviting her to pose for a photo-shoot.
His victim, who had previously done modelling work to help her sister's business, is believed to have died on the day she was reported missing.
Before Reynolds was arrested, media reports claimed he had made advances via Facebook and other social media sites towards Georgia, a former head girl at Telford's Ercall Wood Technology College.
Friends said the A-level student, who was just 5ft 3ins and of slim build, had made it clear she was not interested in a relationship with Reynolds.
Although the facts of the case were not opened at today's hearing, prosecutor David Crigman QC told Mr Justice Wilkie he would have to view distressing photographic material relevant to the case before passing sentence.