Adopted children return to the care system under sec 31 too, because they have been abused/neglected/other by their adoptive parents
Usually the other way round - the child is abusing the parents and/or siblings.
From the 'Beyond the Adoption Order' report, the children who have already left home (by going back into care, or choosing to leave after the age of 16) or very close to the family breaking down:
Anger and aggression during adolescence was a major challenge for adoptive families. Child to parent violence was shown by 41 young people (57%) and was statistically associated with children having left home. Knives were used by 19 children to threaten, intimidate, or control others. Boys were statistically more likely than girls to show child to parent violence.
Parents also described difficulties in coping with teenage children who were oppositional and who showed inappropriate sexualised and attention seeking behaviour. Eleven children (16%) had engaged in serious criminal activity - all but one had left home.
Forty children (57%) had run away or gone missing from home. The police had often been involved in locating the children. Some went to stay with adults whose motivation for befriending the child was questionable and parents feared that children were at risk of exploitation.
Twenty-six children (37%) had made an allegation of abuse against an adult whilst living in the adoptive home. Adoptive parents were most commonly accused, but so too were other family members. The majority of allegations were concluded after a brief investigation by social workers or the police and with no further action.