Governors in Wales lack the knowledge and local authority support to ensure workforce laws are enforced, say teaching unions.
One heads' union also predicts redundancies as hard-up schools struggle to pay for classroom cover under the new edict from England.
Their concerns, voiced at an Assembly committee last week, followed an announcement last month by Ed Balls, England's Schools Secretary, pictured, that schools flouting the 2003 workforce deal could have their budgets suspended and governing bodies sacked.
Under tough new measures, local authorities will have the power to punish schools that fail to comply with the agreement, which says teachers should have 10 per cent of their time set aside for planning and preparation, and should not provide more than 38 hours' cover a year, or carry out more than 20 clerical tasks a year.
But this has angered heads, who believe authorities in Wales are less supportive than those in England in helping school with workload issues. Unions also say governors should not be responsible for ensuring compliance.
Mr Balls is able to get tough over the agreement as Westminster retains power in Wales over teachers' pay and conditions.
Many heads believe the deal is being enforced in most schools in Wales, but enforcement has led to greater workload as they struggle to provide cover.
Education minister Jane Hutt is now in talks with English officials over compliance. She told union representatives at the enterprise and learning committee that she is also working with governors to raise awareness, including overseeing heads' work-life balance.
But Gareth Jones, secretary of the Association of School and College Leaders Cymru, said: "We are in agreement that compliance is important. However, the funding has to be there. There is also a real fear it will lead to redundancies."
He warned that the cost of hiring staff to take lessons currently covered by teachers could mean job losses.
Elaine Edwards, general secretary of the Welsh-medium teaching union UCAC, told the committee that clear leadership was needed from local authorities.
But Rex Philips, Wales organiser for the NASUWT, said schools had had long enough to ensure the agreement was being enforced.