Teachers could be allowed to carry out knife searches of entire classes - rather than just individual pupils - under government proposals.
Carrying a knife can lead to a four-year jail term
A Department for Education and Skills spokesman said Education Secretary Alan Johnson was "minded" to extend powers.
Pupils could not then pass weapons to classmates to avoid detection, he said.
But the Association of Teachers and Lecturers said police, rather than teachers, should carry out searches, as they could be dangerous.
Mr Johnson is said to want to changes to the Violent Crime Reduction Bill, currently going through Parliament, to allow whole-class knife searches.
Schools Minister Lord Adonis told BBC Radio 4's World At One: "Our main reason for doing this is that we believe this would have a powerful deterrent effect.
"The number of cases where this would need to be done, I suspect, would be small, because we don't have a knife culture in anything but a small fraction of our schools."
Safeguards mean teachers, who would receive training, would have to be accompanied by another adult.
Education unions generally welcomed the move.
John Bangs, head of education at the National Union of Teachers, said: "I think it is a good idea in principle, but I think head teachers have to have the power, not the duty, to search for knives and other offensive weapons."
However, ATL general secretary Mary Bousted said large-scale searches simply on suspicion of possessing a knife "will be fraught with difficulties".
She added: "Teachers are not police officers nor are they trained in security work.
"Those schools which do decide to go down this road will need to proceed with caution.
"Many will prefer to benefit from the professional expertise of the police at an early stage."