The Government expects teachers to play a key role in preventing girls as young as 13 from being taken abroad and forced into unwanted marriages.
Foreign Office guidelines will be issued to schools in Wales, England and Scotland next month to highlight the tell-tale signs of pupils at risk.
Hundreds of teenagers are taken abroad every year and married to men they have never met. In many cases, girls are forcibly abducted by their parents and raped by their new husbands.
Such marriages may also lead to enforced pregnancy or abortion.
Warning signs for teachers to watch out for include a sudden loss of academic interest, selfharming, anxious or depressive behaviour, and a history of older siblings leaving education early.
Mal Davies, head of Willows high school, Cardiff, has had to deal with one such case. The situation was resolved after discussion with the family. He welcomed the Foreign Office advice and said it would help schools to deliver a consistent message to families.
"Often youngsters do disclose to teachers their anxieties and their wishes not to have to participate in these arrangements," he said.
"It's a very difficult line for teachers and schools to tread."
Chris Keates, general secretary of the second-biggest teaching union, the NASUWT, said: "Teachers are trained to report changes in behaviour. They are interested in keeping pupils safe, whether in terms of forced marriages, abuse or health issues."
David Grahame, head of the Forced Marriage Unit at the Foreign Office, said he hoped teachers would treat suspected forced marriage as they would other personal problems. "Often, a young person will not think to approach the police, or think it will bring shame on their family," he said.
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