Campaigners have called for urgent action in the wake of a report into the deaths of three children who apparently took their own lives while in custody.
The Prisons and Probation Ombudsman looked at the deaths of Alex Kelly, 15, at Cookham Wood in Kent; Jake Hardy, 17, at Hindley in Wigan, Greater Manchester, and Ryan Clark, 17, at Wetherby, West Yorkshire.
The report by Ombudsman Nigel Newcomen found that two boys had suffered bullying and two had refused to take medication for attention deficit hyperactivity disorder before they died.
Chief executive of the Howard League for Penal Reform, Frances Crook, said the boys' deaths could have been avoided if they had received a non-custodial punishment.
She said: "How many more children need to die before we stop putting them in prison and start realising that tackling their complex problems is more important than harsh punishment?
"The simple fact is that children who end up in custody are highly vulnerable - they might be victims of neglect, have mental health issues or be growing up in a home plagued by drug and alcohol abuse."
Deborah Coles, co-director of campaign group INQUEST, said: "There have been 34 deaths of children in prison custody since 1990. We have helped many of their families through inquest after inquest raising the same issues and, despite promises of change, the deaths continue. It's time to break the cycle of harm and death."
The report said that two of the teenagers should have been moved to specialist units at the Young Offenders Institutions (YOI).
It found: "The three children were extremely vulnerable but, following the court decision to place them in custody, there does not seem to have been sufficiently detailed consideration given to the best placement to help manage their vulnerability.
"In two of the cases, when it became clear that the boys were struggling to cope with a normal YOI regime, they were not moved to specialist units within the YOIs despite alternatives being available."
The ombudsman published anonymous details of the cases on Wednesday and will release more information once inquests are complete.
The report revealed that two of the teenagers, who died in 2011 and 2012, may have been bullied.
It said: "One child reported this bullying repeatedly to a variety of staff. However, he gave no names and without names staff said they were unable to act, even though it was clear who he meant.
"Evidence from CCTV suggested that even when staff witnessed harassing behaviour from other young people it was not adequately challenged.
"In the other case, bullying was not reported and might have occurred only the evening before the boy took his own life. However, bullying by shouting out of cell windows was a recognised problem at the establishment."
Concerns were also raised by relatives of the third child that he may have been bullied, but no evidence was found to back the claims.
The ombudsman said that YOI staff should respond more robustly to bullying and make sure that children have access to outside support such as family members when in crisis.
It also found that mental health assessments did not take into account the different ways that children might show distress.
A Ministry of Justice spokeswoman said: "Deaths in prison are amongst the most scrutinised of all events in custody and every death is subject to an investigation by the police and the Prisons and Probation Ombudsman (PPO), as well as a Coroners inquest.
"The learning points highlighted by the PPO raise important issues and provide a crucial tool for front line staff. Strenuous efforts are made to learn from each death in custody."