Amy El-Keria died after tying a scarf around her neck while receiving treatment at the Priory, which runs mental health services as part of a contract with the NHS in Britain.
The teenager, who had told staff on the day she died that she wished to end her life, was found in her room at The Priory Hospital Ticehurst House in East Sussex in November 2012.
Delivering findings that are highly critical of the Priory, a jury in Horsham said she died of unintended consequences of a deliberate act, contributed to by neglect.
It ruled that staff failed to dial 999 quickly enough, failed to call a doctor promptly, and were not trained in cardiopulmonary resuscitation.
The response of staff was so inadequate that the jury agreed there was a possibility Amy may have lived if she had received proper care.
It said staffing levels were not adequate, and a lack of one-to-one time caused or contributed to Amy’s death in a “significant” way.
Amy, who had a complex range of problems and mental health diagnoses, including attention deficit hyperactivity disorder, Tourette’s, oppositional defiant disorder, gender identity dysphoria and conduct disorder, was moved to the Priory in August 2012 after being asked to leave her specialist boarding school, High Close in Berkshire.
The inquest heard that, while at school, she had drawn a picture of herself hanging and had written underneath: “If only this could happen, but I haven’t got the guts.”
Sylvia Tang, Priory group medical director, said: “We would like to offer an unreserved apology and our heartfelt sympathies to Amy’s family.
“Following the incident, we undertook an extensive investigation and strength-ened a number of our procedures at the hospital.”