A rising number of teenage boys were taken to hospital as the result of "very serious" violent attacks at a prison for young offenders, a new report has found.
Her Majesty's Inspectorate of Prisons (HMIP) said prisoners at Cookham Wood Young Offenders' Institution near Rochester, Kent, had used weapons including sharpened cutlery and items in socks in attacks on fellow inmates.
The number of assaults on staff had doubled to 21 in the six months before March this year, while there had been 61 assaults and 92 fights over the same period.
Many of those needing hospital treatment suffered injuries caused by "attackers jumping on their heads", while during the unannounced visit, inspectors witnessed boys conducting a "kangaroo court" of another boy unchallenged by staff.
Nick Hardwick, chief inspector of prisons, said Cookham Wood reflected "systemic problems" with the prison system for young offenders.
He said: "The welcome fall in the number of children in custody means that those who remain represent a more concentrated mix of very challenging young people, held in a smaller number of establishments that are increasingly unsuitable to meet their needs, and cared for by a staff group beset by shortages and a lack of training for their complex and demanding role.
"This report makes recommendations about what Cookham Wood could and should do to improve, but a much wider political and policy response is needed if we are to fulfil our responsibilities to care for these, our most damaged children safely, and help them to grow into adults who are valued, not feared."
Inspectors found that behaviour management was "weak", and prisoners had "much too little time out of their cells" - with outdoor exercise restricted to 30 minutes a day.
This was linked to a staff shortage, he said, adding that around a quarter of the prison workers "did not know the work, the institution or the boys".
The prison, which held 166 sentenced and remanded boys aged between 15 and 18 at the time of the inspection, has previously been warned about the levels of violence.
A report last October revealed some 35 weapons were found in a lockdown, and 169 acts of violence were recorded during the six months before the inspection.
But the latest inspection found there had been some improvements, including to family visiting rooms, healthcare provision and the levels of self-harm.
Among the HMIP's recommendations were that prisoners spend at least 10 hours out of their cells, including an hour outside, and training and work opportunities should be increased.