An Afghan asylum seeker was taken out of his Croydon school and removed from his foster family after Italian authorities claimed the 15-year-old boy was actually an adult, the High Court has heard.
The man, who has not been named, was deemed to be a "short of stature" adult pretending to be a child.
Judge David Waksman QC ordered the council to send him back to school "if reasonably possible," until his case returns to court.
When he arrived in Britain last summer, the man claimed he was a 15-year-old unaccompanied child.
The High Court heard the migrant issue arose after Italian authorities got in touch, saying he had already claimed asylum abroad and they believed he was an adult.
But despite this, the asylum seeker, believed to be 18, is still receiving all the benefits given to children - until the case has a judicial review.
In December, Croydon Council's social workers carried out an assessment of the man and deemed him "older than 18".
"His demeanour and appearance are suggestive of someone older than his claimed age. He could not give an explanation for how his asylum claim took place in Italy," the report read.
If a judge rules the man was unlawfully detained he could receive up to £4,000 in compensation.
But it is not the first time Croydon Council have been faced with the issue.
In 2011, tens of thousands of pounds of taxpayers' money was spent on establishing the age of an Afghan asylum seeker. The High Court eventually ruled the boy was 18 years old, not 21 as the council had thought.
The boy was told he could claim "former child payments" from the council until he reached 21.
Although there are relatively quick ways of establishing the age of an individual - through dental records - these are not allowed on ethical grounds.
A Croydon Council spokesman declined to comment on the particulars of the case, as it is still ongoing, but said there are "obvious reasons for trying to ensure adults are not placed in the midst of children."
Croydon is home to the UK's only walk-in asylum centre, which is run by the UKBA.