Pupils excluded from school are at a "heightened risk" of being drawn into county lines drugs gangs, an official report warns.
Police officers "reported a strong link between school exclusion and becoming a target for county lines criminals", according to Her Majesty's Inspectorate of Constabulary and Fire and Rescue Services.
Inspectors were not able to establish the extent of the problem but said they were told of more than one incident.
The report notes that police were not informed of pupil exclusions if it was not for a criminal matter.
The report says: "In these cases, the exclusions had not been recognised as warning signs of a greater potential for exploitation.
School exclusions: Pupils falling victim to drugs gangs
"This deprived police and other professionals the opportunity to work with these children and divert them from criminality."
In one instance, a boy who was excluded moved to another secondary school in which he set up a county lines operation and "encouraged other children to join him", the report warns.
Inspectors also raised concerns about how those considered vulnerable who had been drawn into the gangs were handled by police.
The report recommends that councils should tell police when they are moving at-risk children to a different area.
Mark Russell, chief executive at The Children's Society, which is calling on the government to enshrine protections for youngsters in law, said: "Too many children experiencing horrific criminal exploitation through county lines continue to be let down by the frontline services best placed to protect them.
"While we welcome these recommendations and the progress made by police in responding to this worrying situation, all agencies are still failing to consistently identify children at risk and share crucial information.
"Young people too often end up being treated as criminals rather than recognised and supported as victims."
Deputy Assistant Commissioner Graham McNulty, who leads the National Police Chiefs' Council work on county lines, said: "We know there is room to improve and the report has outlined some areas which we are already working towards.
"Police are committed to dismantling violent county lines networks and protecting the young and vulnerable people who are exploited by them, but we can't do it alone.
"Schools, health and social care services, charities and others have a critical role in ending this evil practice and we will continue to work closely with them."
A government spokesman said it was investing £220 million in prevention programmes to support young people at risk of exploitation and the Department for Education was carrying out an "ambitious programme of action on exclusions", which would "ensure excluded children continue to receive a good education".