Wandsworth Police
Serena Alexander-Benson, 13, was last seen by her dad leaving their home in Wimbledon, south-west London, at 7.50am on Friday.
Her father said she was dressed in her green uniform when she told him she was going to school, but failed to show up on the day she vanished.
The Metropolitan Police have said she is understood to have gone through passport control and is thought to be “probably in the company of an older person”.
While the Year 8 pupil lives in the UK with her father, her mother lives in Poland.
In a statement, the Metropolitan Police said: “Active lines of inquiry are being pursued to locate her.
“Anyone who may know Serena's whereabouts or who may have seen her as she travelled through Folkestone - probably in the company of an older person - should call officers at Wandsworth CID on 020 3276 2588.”
Scotland Yard believes the teenager left the country from Folkestone, Kent, on Friday morning and have appealed for anybody who saw her to come forward with information.
Holy Cross School, where Serena attends, also gave a statement.
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They said: “You may already be aware that one of our Year 8 pupils is missing.
“The pupil and her family are in our thoughts and prayers at this difficult time.
“The situation is being managed by the police and we are in liaison with them and the pupil’s parents.”
They added: “If you have any further information please contact the police urgently.”