The Professional Association of Teachers and Unison, the public-sector union, have complained to the Department for Education and Skills that heads are "coercing" assistants and nursery nurses to cover for long-term illnesses and to give teachers preparation and assessment time.
One assistant said she and colleagues had been forced to take art lessons, with no extra pay, so teachers could get their statutory free time to plan lessons and mark work.
"It was worded in a way which meant we really had no choice if we wanted to stay," she said. Another said that teaching assistant colleagues were taking whole classes. "I am so glad that I said no, but it probably hasn't done much for my career prospects," she said.
The PAT was one of the signatories of the national workforce agreement, which allowed schools to use higher-level teaching assistants to fill in for some lessons.
Philip Parkin, general secretary, said the union remained committed to the deal but that it had been hijacked by some schools seeking to save money by forcing staff into roles they did not want or should not have.
He said the agreement said that assistants would only be used for a maximum of one day for cover, that their competence would be checked before taking lessons and that they could refuse to take classes.
"While many of our assistant members are keen to accept opportunities for personal development and additional responsibility, there are also those who do not want to take on a whole-class role and find themselves being coerced into it," he said.
Christine McAnea, Unison's national secretary for education staff, said the practice was fairly widespread. "Our message to members is that headteachers should negotiate any changes to contracts with their support staff and their union. If they don't, just say no."
The Workload Agreement Monitoring Group has set up a sub-group which is examining pay and conditions for support staff and will report in April.
A Department for Education and Skills spokesman said heads must check whether teaching assistants were suitably trained and willing to take classes.
"We are clear that taking on additional roles and responsibilities should not be compulsory for support staff," he said.
PAT is holding an online poll to find out non-members' and members'
reactions to the workforce changes at www.pat.org.uk. A Unison 'survival guide' to school remodelling can be downloaded at www.unison.org.uk