Veronica Sbircea has not been seen since she was spotted in Brixton holding hands with a man on July 28.
She had left her parents and brothers at home in Kettering a week earlier. She spent a week in Northampton before boarding a train from the town to Euston.
Police have said they are extremely concerned for Veronica’s welfare because the prolific social media user has not been on Facebook for some weeks.
Her family has already appealed for her to get in touch, and now her teachers have also joined the calls for her to be back home in time for the beginning of the new school year on Monday.
Amanda Doherty, Kettering Science Academy progress leader for year 10 and 11 said: “Veronica has a bubbly, cheeky manner about her that forms her character, she always has a smile on her face.
“She has a good network of friends in school having started with us towards the end of year nine. She is very chatty and has a good rapport with staff and students alike.”
KSA Principal Paul Davies said: “Veronica is a very popular and well liked student at the school. Although she has only been here a relatively short time she has played a full part in our school community.
“We are obviously very worried about her disappearance and would urge anyone with any information as to Veronica’s whereabouts to get in touch with the police.”
The youngster came to the UK in 2008 with her parents and two older brothers. The family settled in London but came to Kettering last year for a fresh start.
Although Veronica has been briefly missing before, this is the first time she has been gone for a sustained period of time.
A team of 16 officers from Northants and from the Metropolitan Police are searching for her.
Anyone who knows where she is, or has heard from her, can call Northamptonshire Police on 101.