They were found seriously unwell in a street and taken by ambulance to hospital for emergency treatment.
Police officers then examined digital devices to identify the name of the online group and its other members.
Seven of the 12 girls had self-harmed prior to being traced by the police. Children’s social care services from seven different local authorities have been involved in safeguarding children identified as members of the group.
Police said in a statement to BBC that “peer-to-peer influence increased suicidal ideation amongst the children involved to the extent that several escalated to suicidal crises and serious self-harm.”
Instagram says it found no evidence of its rules being broken as it uses Artificial Intelligence (AI) to hunt and block self-harm posts and groups.
Facebook, which owns Instagram, does not deny that the name of the closed group referenced “suicide” but says it has not been removed from the platform because the content of the messages does not break its rules.
In a statement, a company spokesperson said it was co-operating with the police.
“We don’t allow graphic content, or content that promotes or encourages suicide or self-harm, and will remove it when we find it.
“We’ll continue to support the police and will respond to any valid legal request for information.”