By Justin Parkinson
BBC News education reporter, at the NASUWT conference
Britain's second-biggest education union is writing to hundreds of head teachers warning them not to ignore a government deal on staff workload.
Teachers' leader Chris Keates gave a warning to heads
The NASUWT claims a tenth of heads in England and Wales are ignoring the workforce agreement, which reduces teachers' commitments.
General secretary Chris Keates said this attitude showed "desperately ineffective management".
But a heads' leader said lack of cash was keeping many awake with worry.
Ms Keates said: "The law is the law. Heads are not above the law and we are not about to have our members denied their working conditions."
The workload agreement, signed by most of the main education unions, states that teachers should not carry out jobs like reading the register and supervising playtime.
From September, teachers are also entitled to time out of lessons for marking and lesson preparation.
The National Association of Head Teachers has decided to pull out of the agreement, saying there is not enough money to provide extra classroom cover.
NAHT president Dr Rona Tutt told the BBC News website: "Everyone wants to implement the workload agreement and believes it is a good thing.
"But our members' vote to leave it reflected the fact that they hadn't got the resources necessary. Many are lying awake at night, seriously concerned about not having the funding.
"You can argue about how much money the government has paid in centrally, but funding is different by local authority and by school."
Law of the land
Ms Keates, speaking at the 223,000-member NASUWT's annual conference in Brighton, said 10% of head teachers had done nothing yet towards implementing the agreement.
But previous research had shown that all schools could work out budgets to do so.
She added: "If a teacher went into a school and simply said 'I'm not obeying the law of the land and not following the contract', the governors and the local education authority would be exhorting the head teacher to take the harshest teacher discipline procedures."
These would be gross misconduct charges, leading to dismissal.
Ms Keates called upon governors to discipline heads, to avoid a large number of court cases against them.
The agreement would allow more "focus on teaching and learning", while some heads had "equated status and management with divorcing themselves from the classroom and focusing on administration".
The government was backing the NASUWT's letter-writing campaign, which also targets governors and local education authorities, Ms Keates said.
The National Union of Teachers - the only big union not to sign up to the agreement in the first place - has voted for industrial action ballots over its implications.
Its main concern is that classroom assistants might take lessons rather than qualified teachers and it wants to ensure changes are properly funded.