Headteachers have taken legal advice on the threat of action from teachers and could face a "very difficult autumn" unless they implement the workload agreement, their leader said this week.
However, David Hart, general secretary of the National Association of Head Teachers, said it was "fanciful" to suggest that hundreds of heads will be hauled before the courts for failing to introduce workforce remodelling.
And he warned the classroom unions that a consequence of implementing reforms could be job losses among staff.
Mr Hart said the union had consulted lawyers over threats issued last week by the National Association of Schoolmasters Union of Women Teachers. He said: "We are taking legal advice to identify areas where action is possible and we will be presenting a paper to our national conference which will identify the way forward on workforce agreement and funding issues."
Today, Mr Hart's successor, who will inherit the headache of the workforce war, will be announced. The leadership's preferred candidate is David Hawker, Brighton and Hove's director of children's services. He has been challenged by Mick Brookes, a Nottinghamshire primary head and former president of the union.
According to advice received by the NASUWT, teachers have several avenues of legal recourse:
* judicial review to test how the arrangements are being implemented;
* employment tribunal if it can be proved, for example, that more women than men were being penalised;
* county or high court action over a breach of working time regulations.
This is a criminal offence which could result in a fine or custodial sentence.
Individual teachers could also take civil action for stress and related illnesses. And the Government could issue a further sanction, called a writ of mandamus, reinforcing the law.
But Mr Hart said that governors and local authorities, as employers, would also be liable because they will have sanctioned the heads' actions.
He is convening a meeting on April 21 of the union's senior officers to discuss the matter, before the annual conference which begins on April 29.
Mr Hart said: "The action that can be taken against heads is far more limited than the NASUWT would have us believe. The only way to get at a headteacher is to persuade the local authority to withdraw their delegated budget, which an LEA is unlikely to do."
In schools where there were budgetary constraints unions might have to face the consequences which could be redundancies, he said. "It would be quite stupid to diminish the seriousness of the situation. It will be a very difficult autumn for a number of schools arguing their case.
"We would be in a much stronger position to negotiate if we remained part of the agreement. If this does come to fisticuffs with other unions, we will support our members, but we cannot tell them everything will be all right, because there is no guarantee."
Mike Huss, senior consultant at Peninsula, an employment and health and safety consultancy, agreed that action against heads would be hard to instigate.
However, constructive dismissal was one way to bring the matter before a tribunal. "It would have more impact to have all the staff resign, claiming constructive dismissal, rather than having one sacrificial lamb," he said.
Chris Keates, the NASUWT general secretary, said the NAHT should "stop looking for loop-holes in the law". "In law we would, of course, have to go to the employers and by doing so we will be putting pressure on heads to comply," she said.
The result of the NAHT leadership election will be posted on the TES website, www.tes.co.uk, as soon as it is known.