A 14-year-old girl suffered an accidental death contributed to by neglect while under the care of the Priory, an inquest jury has ruled.
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.
The teenager, who had told staff on the day she died that she wished to end her life, was found in her room at 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 (CPR).
The response of staff was so inadequate that the jury agreed there was a possibility that 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.
Risk assessments were not properly carried out, staff did not assess the risk of her being able to take her own life in her room and opportunities were missed to remove the scarf from Amy, all causing or contributing "significantly" to her death.
The jury also said the Priory failed to properly deal with the fact that Amy was being bullied, and staff failed to share details of the times the teenager had said she wanted to kill herself.
A delay in checking on her on the evening she died also contributed significantly to her death, while she should have been under even closer scrutiny, it said.
The jury, sitting at Horsham Coroner's Court, heard that staff were not trained in resuscitation techniques, despite one healthcare assistant asking for training, and did not always tell parents when their children were being forcibly sedated.
Amy, who had a complex range of problems and mental health diagnoses, including attention deficit hyperactivity disorder (ADHD), Tourette's, oppositional defiant disorder (ODD), 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."
Several attempts to end her life followed throughout early 2012 before Amy was admitted as a "nervous" inpatient to Ticehurst House, following a referral by West London Mental Health NHS Trust.
She was deemed high-risk and put on 15-minute observations, and forcibly sedated on at least two occasions.
On October 27, a football scarf was seen in her room but staff from the Priory admitted at the inquest that it was not taken away and the hospital had no list of banned items.
Just over two weeks later, on the day she died - with her risk rating now downgraded to medium - Amy told a member of staff she wanted to kill herself.
Later that evening, a member of staff found her door locked and realised Amy had decided to try to end her life.
The inquest heard that, before an ambulance arrived, an oxygen mask was put over Amy's face which did not fit and there was a "high level of anxiety among the team".
She was not resuscitated properly by staff, was vomiting profusely and had to be removed from the hospital on a body board because the ambulance stretcher would not fit in the hospital lift.
Earlier in the inquest, Priory staff said that, due to pressure on wards, they had not always been able to give the teenager one-to-one time.
The jury also heard from West Sussex senior coroner Penelope Schofield, while summing up the evidence, that one member of senior staff had "put on training on Tourette's but nobody had attended".
A more junior member of staff had asked the ward manager for CPR training "but had got no response".
Dr 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 strengthened a number of our procedures at the hospital.
"We will now review the findings of the inquest very carefully and consider whether further improvements can be made including in relation to staffing, care plans and risk assessments.
"Since the incident took place, we have been re-inspected by the independent Care Quality Commission which has confirmed the hospital is meeting all national standards."