A report by the Care Quality Commission, Her Majesty's Inspectorate of Prisons and Ofsted into conditions at Rainsbrook STC in Warwickshire also revealed that use of force was high, safeguarding systems not in place and an over-reliance on inexperienced staff.
The Howard League for Penal Reform has called for Rainsbrook, which is run by MTCnovo, to be closed.
Since the last inspection of Rainsbrook in March 2016, the number of violent incidents had doubled. In the six months leading up to the inspection in June, there were nearly 500 violent incidents involving staff and young people.
"This equates to an average of 40 assaults against young people, 36 assaults against staff and seven fights each month," the report states.
While most incidents of violence during this period were low level, 42 young people and 22 staff sustained injuries requiring medical treatment. At least one member of staff and two young people required hospital treatment following an assault.
The inspection also uncovered use of force and restraint had tripled since the previous inspection, averaging around 90 incidents each month in the previous six months since June.
Inspectors reported seeing incidents where young people were relocated to sterile rooms without any personal possessions, and in one case a mattress was removed.
"This is punitive and risks increasing vulnerability," inspectors said. "There was no evidence that any consideration was given to the impact on young people's mental state when placing them in bare rooms."
It found the centre's workforce was "largely very inexperienced" due to vacancies being filled by new applicants.
From October 2016 to May 2017, more than 100 new operational staff started and 74 left.
"This means that the majority of staff working directly with young people on a shift will have less than a year's experience and many have only a few months' or weeks' experience," the inspectors added.
Frances Crook, chief executive of the Howard League, said: "Last month, Her Majesty's Chief Inspectorate of Prisons declared that there was not a single jail in the country that was safe for a child. This is an observation that the Howard League has been making for years, and it surely begs the question of why children are still being sent to places like Rainsbrook, where violence is rife.
"The Howard League opposed the creation of secure training centres in the 1990s and warned that children would be damaged and hurt in these institutions. A long line of inspection reports has underlined that this is a failed model of detention. It is time to put an end to this."
MTCnovo, a partnership between US correctional facility provider MTC, criminal justice charities and UK-based firm Amey, was awarded a five-year contract to run Rainsbrook in September 2015, taking over from G4S Care and Justice Services in May 2016.
Last year, campaigners raised concerns about the suitability of a company that runs US jails to manage a youth custody unit in England.