Inspectors have raised "serious concerns" about safety at one of the country's young offender institutes.
Bullying is common place and staff members have been attacked at Cookham Wood prison, according to a report.
The report said the prison had improved since a 2009 inspection, which found inmates hiding in their cells and rampant vandalism, but conditions were still not "sufficiently safe".
It stated that although there had been improvements from a "very low base", there remained serious safety concerns.
In the four months before the inspection there were three serious assaults on staff members and another two after the inspection took place.
Staff still resorted to the use of force to control prisoners and bullying remained "a significant problem" in the prison.
The inspection criticised the number of inmates being held at Cookham Wood, which has returned to the highest possible capacity of 143 young men, aged 15-to-17, following the previous inspection's demand that the numbers be cut back.
Nick Hardwick, chief inspector of prisons, said Cookham Wood needed "intensive support" to continue making progress, especially in creating a strong group of permanent staff.
"The establishment is safer than it was when we carried out our last inspection, but staff and management need support and stability to build on that to deliver consistently effective relationships with young people," he said.
"The safety of the prison will depend on these relationships, and they are the key ingredient in helping the young people move to law-abiding and useful adult lives."
Prison reform groups pointed to the endemic violence at Cookham Wood as evidence that young offenders were not being rehabilitated in prison.
Frances Crook, director of the Howard League for Penal Reform, said too many children were being sent to prisons where violence contributed to reoffending rates.
"The report into Cookham Wood prison makes a very worrying read. Children sent to prison are bullied by other children, painfully restrained by adult staff and abandoned by society," she said.
"There is a systematic failure to protect children for whom prison is an extension of the abusive and neglectful homes in which they grew up. The state should not be perpetuating the cycle of abuse by sending children to prisons that are dangerous, inappropriate and unsafe.
"Children in the criminal justice system require concerted intervention and family support. A short stretch in a prison and then put straight back where they came from is leading to total failure and guaranteeing a career of crime."
The report comes at a point when justice secretary Kenneth Clarke has committed to reforming the prison service and outlined policies to reduce the size of the prison population.