The "disturbing extent" of self-harm and violence in some young offender institutes "illustrate why prison is no place for a child", a charity has said.
Parc Prison in Bridgend has the highest rate of self-harm among youth jails in Wales and England that house children between 15 and 17, new figures show.
Wetherby had most self-harm incidents while Werrington had the highest total number of assaults on young people.
The Howard League for Penal Reform described the statistics as worrying.
"These figures illustrate in graphic detail why prison is no place for a child," said Andrew Neilson from the charity.
Parc Prison director Janet Wallsgrove insisted improvements had been made.
According to a Freedom of Information request by Cardiff University's Wales Governance Centre, Parc's youth wing had 64 incidents of self-harm in 2017.
The Welsh facility also had the highest rate of assaults against young people, recording 113 incidents in 2017.
Wetherby YOI in Yorkshire had the highest total number of self-harm incidents with 146 while Werrington in Staffordshire had the highest total number of assaults on young people with 209.
"We are failing children at a crucial juncture in their lives," added Mr Neilson.
"If we lock them up and expose them to such high levels of violence, making it more and not less likely that these young people will go on to commit further crimes on release."
Parc's young offenders' unit is attached to a category B men's prison.
Earlier this year, the Welsh Affairs Select Committee heard that Parc was looking to recruit behaviour analysts to deal with some of the violence and self-harm problems.
Self-harm rate in Young offender institutions in Wales and England (per 100)
- Parc, Bridgend - 160 (average population of 40)
- Cookham Wood, Kent - 43.1 (average population of 144)
- Feltham, Middlesex - 27.1 (average population of 129)
- Werrington, Staffordshire - 102 (average population of 98)
- Wetherby, Yorkshire - 79.1 (average population of 184)
"The data presented here reveal the disturbing extent of self-harm and violence being experienced by children in custody," said Dr Robert Jones, from the Wales Governance Centre at Cardiff University.
"These latest figures raise urgent questions over safety levels at Young Offender Institutions in England and Wales including HMYOI Parc, a children's unit in Wales situated within one of the largest prisons in the UK."
Parc Prison's Independent Monitoring Board said "further analysis" of the figures were needed.
But the prison's director Ms Wallsgrove said: "These figures are nearly a year old, and since 2017 incidents of self-harm at YOI Parc have fallen significantly by 80%, and are lower than at comparable establishments."
"Our dedicated staff treat the young people in our care with respect, challenge inappropriate behaviour and ensure a broad range of educational and purposeful activities are available."