Young offenders may be challenging, disruptive and guilty of serious offences but "you feared for them all", the Chief Inspector of Prisons has said.
Nick Hardwick, reporting on Wetherby young offenders institution in West Yorkshire, described how one young boy tearfully asked to be taken home to his mother while another, described as "low", lay on his bed not speaking.
Some of the most challenging of the 340 boys held at the time of the inspection, most of whom were aged 16 and 17, were also the most vulnerable, he added.
"Walking round the establishment, the vulnerability of some of the young people held was obvious," Mr Hardwick said. "One boy in the segregation unit with a lifelong medical condition that would have been hard for any teenager to manage, and who had exhibited very disruptive behaviour, asked me tearfully if I could take him home to his mum.
"I was later told he had been moved to a more appropriate secure medical facility. Another boy, who looked about 12 and was sporting a dramatic black eye, had been convicted of a serious offence, had been in further trouble and was confined to his cell.
"A boy in health care, described to me as 'low', lay on his bed not speaking. All these boys were receiving good attention and care, but you feared for them all."
The inspectors also found a group of cleaners who, "comfortably ensconced in a store room, appeared very pleased with themselves and needed better supervision".
Penelope Gibbs, director of the Prison Reform Trust's Out of Trouble programme, said the Keppel unit at Wetherby offered the only specialist unit for particularly vulnerable boys in the country.
"It cannot meet the needs of all the vulnerable boys in this one prison, let alone all those imprisoned in England and Wales," she said. "This poses a challenge to the whole system, and to the appropriateness of imprisoning any under-18 year olds in a prison service establishment."
Michael Spurr, chief executive of the National Offender Management Service, said: "I am pleased that the chief inspector recognises the positive work being done at Wetherby with a challenging population of young people. The governor and her staff are rightly commended for their care and commitment and they will use the recommendations from the report to further improve and develop the work at Wetherby."