George MacBride, the institute’s education convener, countered that individual members could expect action in the sheriff court from parents if they refused to teach pupils deemed unruly. They could also be in breach of contract.

Mr MacBride, a Glasgow learning support teacher, said that any move towards “scapegoating children” was not the answer.

Ian McCalman, vice-president and a secondary teacher in the east end of Glasgow, said it gave him no pleasure to speak in a debate which exposed union weaknesses. Refusing to teach pupils would put members in “a dangerous situation”. Verbal abuse for some staff might be regarded as “normal backchat” in his school and individuals could potentially hold the union to ransom by taking random action.

Proposing a firmer line, Maureen Watson, North Lanarkshire, argued that discipline has deteriorated over the past five years and school managers were “scared to take the lid off”. Solutions were related to smaller classes, more individual teaching and better resources.

Delegates did agree that teachers should be informed if pupils transferred to their school had a history of bad behaviour. Kathie Finn, a Glasgow primary teacher and leading figure in the union, said one headteacher had put staff at risk by failing to pass on such information.

The pupil had twice tried to burn down the school, stabbed a teacher with a pair of scissors, attempted to strangle a teacher, stabbed his mother in the back, and slashed his sister’s leg from knee to ankle with a long carving knife. Teachers had a right to be aware of such details and to protect themselves, Ms Finn declared.

Bob Dow, Glasgow, warned: “The days are long gone when a teacher could drop a pin and everyone could hear it.”

Willie Hart, Glasgow secretary, said the union should ignore the “populist rant” of the NAS but security was an issue. Guidelines were the best way forward.