A self-harming pupil was given a disposable safety razor to slash themself with at a school while being supervised by a teacher.
The incident happened at Unsted Park School in Godalming, Surrey.
The school provides specialist education for seven to 19-year-olds with Asperger's, higher functioning autism and associated disorders.
The Department for Education said the incident, in January last year, was "deeply worrying".
The pupil did not need hospital treatment.
Self-harm happens when a person injures or harms themself on purpose. Such action may be a sign that something is seriously wrong.
A spokeswoman from the Priory Group, which runs the school, confirmed that the incident had taken place. No details about the nature or extent of the pupil's injuries were given.
The spokeswoman said: "This was a short-term, local procedure introduced by the head teacher and school principal who genuinely believed it was in the best interests of the pupil.
"However, they accept that the procedure should not have been implemented without further approvals having been obtained from key stakeholders and senior management prior to its introduction."
She said the school was always willing to review cases with the Teaching Agency, which is responsible for the recruitment, initial training and supply of teachers.
The spokeswoman said the details of where the pupil harmed themself on their body would be in confidential records.
A Department for Education (DfE) spokeswoman said: "This is a deeply worrying [incident].
"The DfE commissioned an emergency inspection by Ofsted as soon as we became aware of it, which was followed by a full inspection of boarding welfare in December 2012.
"Both inspections found some failings which we are currently working with the school to rectify."
The Priory Group spokeswoman said the failings were not related to the razor incident.
A spokeswoman from selfharm.co.uk, which supports young people affected by self-harm, said young people self-harmed for "any number of reasons", including in response to emotional stress.
She said: "The best way to help is to listen without judging, accept that the recovery process may take a while and avoid 'taking away' the self-harm.
"Self-harm can be about control, so it's important that the young person in the centre feels in control of the steps taken to help them learn to manage without needing to hurt themselves.
"Self-harm affects as many as one in 12 young people, but recovery is always possible."