Inspectors found makeshift weapons and pornography at the Oakhill youth detention centre in a recent inspection.
Ofsted found there had been an increase in the number of banned items discovered at the secure training centre in Milton Keynes, which included makeshift weapons and unauthorised USB sticks.
The report said: “Some USB sticks found in young people’s rooms contain music, but others contained 18-rated films and pornography. The source of these items is unknown.
“The centre’s own findings demonstrate that staff are not identifying and appropriatelydealing with prohibited or restricted items in their daily work when some of these items are obviously present in communal areas or bedrooms.”
The independent watchdogs said searching of staff and visiting professionals at the facility is ‘very regular’ but ‘inconsistent’.
The overall report stated the secure training centre ‘requires improvement’ and was rated ‘inadequate’ for promoting positive behaviour.
Oakhill is managed by G4S and offers secure accommodation for up to 80 males aged between 12 and 18 who have been sentenced or remanded to custody.
The Ofsted report found the centre does not have an ‘effective or consistent’ approach to managing inmates’ behaviour.
This had led to a rise in inappropriate conduct, and inspectors witnessed some staff on some units struggling to maintain order and control.
Levels of violence were described as ‘very high’, with more than 200 assaults on staff or detainees between July and December.
Violent incidents had resulted in 54 injuries to staff and inmates, with seven serious enough to require hospital treatment.
The report said an ‘inconsistent’ approach to violent episodes meant some perpetrators ‘faced little or no consequences’.
However, the achievement of young people has improved and is rated as good.
G4S announced last year that it was to sell its children’s services division, including the contract for running Oakhill.