The UK's largest teachers' union has warned government not to introduce measures that would make teaching staff criminally responsible for failing to report the forced marriage of a pupil.
A Home Office consultation is exploring such proposals for teachers, health workers and social workers.
But the National Education Union (NEU) said it "raises the stakes" without the required training for its members.
The consultation's findings are to be published "in due course".
The government is aiming to find new ways to increase reporting and clamp down on forced marriage.
Since it was made a criminal offence in 2014, there have been only four convictions in England and Wales, figures provided by the Home Office show.
But Amanda Brown, the NEU's deputy general secretary, said a law that could make teachers criminally responsible was the "wrong approach".
"The idea of criminal sanctions does raise the stakes for professionals but without [supplying them with] the training, knowledge and understanding," she said.
The union is against any form of mandatory reporting.
"We need a longer-term approach, ensuring... that young people understand they can raise their concerns with their teachers," Ms Brown added.
"Hana" - not her real name - ran away from home a year ago, aged 18, and now lives in hiding.
During her A-levels, her mother told her she was going to marry her first cousin, whom she had never met.
"I said that I wanted to marry someone that I loved but she kept telling me that getting married within the family would be good," Hana says.
"I had anxiety attacks in school and my teachers would often ask, 'Is something going on at home?'
"But I didn't want to tell them that maybe I'm worried about marriage, that I can't sleep at night [because I'm] just thinking and thinking about getting married to someone I don't know."
So Hana decided to flee.
Asked why she had not told her teachers, she said: "I thought maybe they'd involve people I didn't want involved in this situation, like social services or the police."
In Birmingham - where there is a relatively high number of forced marriages - police say they work closely with teachers, training them on how to spot signs a young person is at risk.
Det Insp Wendy Bird, of West Midlands Police, said there was often just "one chance" for them to intervene and take the necessary safeguarding steps.
"In the worst cases, people are murdered as a result of turning down forced marriages, because it's [seen as] dishonourable," she added.
Bev Mabey, head of the Washwood Heath Multi-Academy Trust, in Birmingham, told the BBC's Victoria Derbyshire programme: "Girls would often ask us to hold their passports - so that when it came down to it, there wasn't actually a way of leaving the country."
But, she added, when police and social services had become involved, on "one or two occasions" the girls had been removed from their family.
One organisation that has been campaigning for mandatory reporting is the Freedom Charity, although it does not believe teachers should be made criminally responsible.
Its founder, Aneeta Prem, said it was telling the Home Office "very clearly that we think there should be mandatory reporting, especially for teachers to come forward and report any of the signs of forced marriage".
"Teachers play a huge and important role. Kids go missing in GCSEs, so they can save lives," she added.
The Home Office said in a statement that its consultation "was open to everyone and encouraged victims and survivors of forced marriage, those with expertise in the area of forced marriage, and relevant professionals to come forward".
"We will give careful consideration to all the responses received," it said.