An Afghan asylum seeker spent months at a Croydon secondary school until he was deemed to be a "short of stature" adult acting younger than his years.
The 5ft 4in migrant arrived in Britain last summer and claimed he was a 15-year-old "unaccompanied child," the High Court heard.
He was placed with foster parents by Croydon Council and sent to school, but was pulled from both three months later when it emerged authorities in Italy, where he had also claimed asylum, believed he was an adult.
But the pint-sized Afghan has now forced Croydon Council to continue looking after him as a child, pending a full judicial review of his case.
He could receive up to £4,000 compensation from the Home Office if it turns out he was "unlawfully detained."
Despite Croydon Council's insistence he is a grown up, Judge David Waksman QC ordered the council to send him back to school "if reasonably possible," until his case returns to court.
The High Court head the mystery migrant was quickly put with the foster family and given a place at school last summer, where he claimed to have fled Afghanistan following the Taliban's kidnap of his father.
But his fingerprints matched those taken by Italian authorities last year, where the same migrant gave his date of birth as January 1993, making him 18.
Croydon social workers carried out their own age assessment in December and deemed him "older than 18," resulting in his removal from school and foster care and him being earmarked for removal to Italy.
The age assessment report said: "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."
It was also found he showed little emotion when describing his father's abduction, was more confident than an average child would be and showed signs of needing to shave.
But his lawyers claimed social workers failed to talk to teachers at his school, or his foster parents and ignored the fact he was only 5ft 4in tall.
Judge David Waksman QC ruled "arguable" claims that the age assessment did not meet the minimum legal standards and ordered a full judicial review of the case later this year.
The migrant now at liberty, was due to be sent to Portsmouth but will now stay in south London and receive all the benefits due to a child, pending the hearing of his case.