The Educational Institute of Scotland has accused Glasgow of failing to face up to indiscipline in schools and warned that it may take "unilateral action". Glasgow denies the charges.
Willie Hart, EIS secretary in the city, issued the threat and went so far as to claim the council restricts the definition of "a serious incident" to such an extent that it prevents action being taken.
"It appears to preclude any behaviour, no matter how outrageous, that does not involve committing acts of violence or drug-related matters," Mr Hart said. "Verbal abuse, threats of violence, and persistent disruption and defiance are apparently not serious incidents."
The latest flare-up comes days ahead of a Holyrood debate on indiscipline next Wednesday in which ministers are expected to refute claims from opposition politicians that some schools are becoming "battlegrounds".
The EIS concerns were aired at a closed meeting of members on Tuesday which heard from Alana Ross, a former national president of the union, that indiscipline is "the most pressing issue in Scottish education".
In an unusual move, Gordon McKenzie, former president of the Headteachers'
Association of Scotland, was invited to address the meeting. Mr McKenzie served on the Scottish Executive's discipline task group with Ms Ross.
He told The TES Scotland later that Glasgow was not alone. "The concerns I have heard tonight would be repeated across the country because discipline remains an ongoing concern for teachers, affecting morale, recruitment and retention."
The task group report was "very sound", Mr McKenzie said, but its recommendations had to be implemented without delay, in particular the need for multi-agency support.
The union is calling on Glasgow to participate in a joint survey into the extent of the problem and for the removal of "unreasonable obstacles and much informal pressure not to remove seriously disruptive pupils from school".
Some members are prepared to go further. Anne O'Donnell, an English teacher at Hillhead High, said: "We should be more proactive and draw up our own rules about what is regarded as acceptable because present arrangements are not working. Teachers wonder about the point of reporting incidents if they don't see effective action being taken."
An experienced teacher of special needs pupils said that during the past year she reported 30 incidents in which she had been "punched, slapped, kicked, butted and was nearly strangled".
A spokesperson for Glasgow said that two categories of behaviour can lead to exclusion - a "serious incident" involving violence or drugs and wider grounds of "serious concern" which cover persistent disobedience and threats.