'Mob attacks and serious assaults' at Cookham Wood youth jail
More than 100 violent incidents were recorded at a young offenders' institution in Kent in the six months before a visit by inspectors.
Mob attacks and serious assaults on staff, some involving gangs of boys attacking a single young person, happened at Cookham Wood in Rochester.
HM Inspectorate of Prisons said assaults on staff had risen by nearly a third since the last inspection.
Overall, inspectors found the facility was well-led and working effectively.
Chief Inspector of Prisons Peter Clarke said the institution was continuing to improve, with "difficulties, risks and weaknesses" being attended to in "effective and often creative and innovative ways".
"The prison was led with confidence; the management team seemed cohesive and attentive and an evident strength was the quite impressive culture that was developing among the staff as they grew in both experience and confidence," he said.
Michael Spurr, chief executive of the National Offender Management Service, added: "There remains more to do, particularly on safety, but work is under way to address this, including the introduction of a new behaviour management strategy and a new unit dedicated to supporting the most challenging offenders."
He praised the professionalism and commitment of staff, and said "the strong foundations now in place" would allow the prison to drive further improvements over the coming months.