Teaching assistants who are being paid as little as pound;3.50-an-hour extra to stand in for teachers were this week told to refuse to cover lessons.
Wages for many of the new breed of "highly-trained" assistants are only increased for the time spent on classroom cover - effectively putting them on two contracts. Assistants will have a key role in implementing the Government's workforce agreement, which gives teachers half a day every week outside the classroom for marking or planning.
More than 4,000 higher-level teaching assistants (HLTAs) are expected to be trained in England by the end of this term, and another 10,000 people have registered for courses. Only around 150 are expected to be accredited in Wales by the autumn.
But Unison, the country's biggest support staff union, said thousands of schools were preparing to exploit the newly-trained staff by only paying the improved rate - often pound;3.50 an hour more - for time actually spent standing in front of classes.
The issue is set to become a flashpoint at the union's annual local government conference in Glasgow next month, when delegates will push for national pay scales to eradicate inequalities.
Christina McAnea, the union's head of education, said: "HLTAs are trained in 31 different roles - standing in front of a class is only one of them.
It is completely unacceptable if schools are only prepared to pay at the higher rate for one or two hours a week.
"Where this is happening, Unison's advice is to boycott the HLTA role altogether."
Unison delegates in the West Midlands will tell next week's conference that the role of HLTAs is being "undermined by employers not applying the grade throughout the week. Members find themselves with two contracts".
It follows a TES survey of 545 schools last week which revealed that most will not be able to pay for the cover needed to provide staff with a half a day off. Currently, each local council sets wage levels for teaching assistants, averaging around pound;13,500 for normal duties and between pound;18,000 and pound;22,000 for HLTAs. Unison has drawn up proposed new national pay scales, which would see normal teaching assistants paid up to pound;15,015 and higher-level assistants between pound;23,265 and pound;26,157, more than the pound;19,161 starting salary of a Welsh newly-qualified teacher, noted Anna Brychan, director of the National Association of Head Teachers Cymru.
"Fundamentally this is a budget problem," she said. "We would support funding levels that allow heads to employ whomever they think best to raise standards in their school, and to pay them honourably for doing that."