Teachers are being forced to spy on their students over fears about Islamic extremism, a teachers' union conference has heard.
New counter-terror rules aimed at exposing radicalisation require schools to report suspicions about pupils to the police.
The National Union of Teachers says the rules are stifling debate in schools.
The Conservatives said the guidance made clear that no teacher should feel unable to talk about sensitive issues.
A party spokesman said teaching about "fundamental British values" actively encouraged such discussions.
But the NUT conference in Harrogate heard how teachers were becoming too nervous to discuss controversial issues such as Islamic fundamentalism.
One Wandsworth teacher, Jan Nielsen, said: "We are expected to be front-line stormtroopers who listen, spy and notify the authorities of students who we are suspicious of."
She told delegates of a Muslim pupil who had been questioned over comments he made after Friday prayers, even though he had made a strong argument against extremism.
And she gave a case of another boy being questioned after he applied for permission to visit his dying grandfather in Pakistan.
His laptop was seized and he was accused of looking at jihadi websites by the headteacher.
In response the boy said: "How can I argue against something if I don't understand it?"
Other teachers claimed Muslims were being demonised and turned into suspects.
'Shutting down debate'
Executive member Alex Kenny told delegates the union was already hearing that teachers did not know whether to open up discussions on such issues because they did not know "where they would go".
He added: "Prevent is shutting down debate and we must oppose it. Schools are places where teachers and children should be allowed to have discussions."
The "Prevent" strategy is part of the government's counter-terrorism strategy, designed to discourage people from supporting terrorism.
In certain circumstances teachers could also face prosecution if they failed to alert the police to any concerns they have about pupils.
Schools must also actively promote "British values" and are judged on how well they teach these as part of the school inspection process.
Delegates agreed a motion which said: "The government's promotion of British values, the Prevent agenda and the use of Ofsted to monitor these is having the effect of closing down spaces for such discussion and that many school staff are now unwilling to allow discussions in their classroom for fear of the consequences."
Speaking ahead of the debate, NUT general secretary Christine Blower gave the example of how teachers had felt conflicted when dealing with the issue of the attack on the French satirical magazine, Charlie Hebdo.
"When the appalling attacks took place at Charlie Hebdo, quite a few teachers said pupils were bringing it into class saying it is a big news story - what is happening here?," she said.
"After the attack, some students, particularly some Muslim students, said they felt if they expressed that they were offended by the cartoons, they would be labelled as extremist.
"The idea that young people themselves are shutting this down means that they are locked out of the discussion."
She said the way to deal with such controversial matters was "to have proper discussions within a robust framework".
And she argued that if any concerns about extremism or radicalisation among pupils emerged in schools, they should be dealt with under existing safeguarding procedures in schools.
Kalsoom Bashir of counter-extremism organisation Inspire agreed that early intervention was key.
She said: "Prevent really is about working with people whether they're members of the community, as mothers we work with, or with teachers, about the need to have open and frank conversations - and recognise when somebody maybe at risk of going down a criminal path and start having those conversations very, very early."
A spokesman for the Conservatives said: "The battle against extremism begins at school where young people learn to be active, resilient and tolerant citizens, ready to seize the rich opportunities of modern Britain.
"Teaching about the fundamental British values of democracy, the rule of law, individual liberty and tolerance and respect for others is part of our promotion of British values and is at the heart of what every school has to deliver for children.
"We want all schools to promote these values throughout the curriculum, not just as a bulwark against extremism, but as a vital part of preparing young people to get on in life. It helps to open young people's minds, making them into citizens who respect difference, who welcome disagreement and who challenge intolerance."