A third of teaching assistants are working up to four hours' unpaid overtime per week, a survey suggests.
Some heads may rely on classroom assistants to cover PPA time
More than one fifth of teaching assistants said they worked between four and 10 hours' unpaid overtime per week, according to the survey.
The public sector union Unison questioned 3,688 local government workers on their pay and conditions.
It claims the goodwill of support staff is being expoited under changes which give teachers more time out of class.
Only Chief Officers worked more overtime than school support staff, the survey found.
Unison national secretary for education staff Christina McAnea said: "The goodwill of support staff is being exploited."
The amount of overtime worked by teaching assistants - which includes learning support assistants - was almost identical to that of nursery staff.
Over half of nursery staff said they worked overtime - 34% working more than four hours' overtime per week, with 23% saying they worked between four and 10 hours.
Almost a third of teaching assistants said they did not carry out overtime.
Teaching assistants were also likely to be dissatisfied with their pay, the survey suggested, with 72% saying they did not believe they were paid fairly for the work they did.
A teaching assistant on the highest level - level four - can earn up to an average pro-rata salary of £23,313 per year, but many are paid on a termly basis only, Unison says.
A teaching assistant on the lowest grade could earn a pro-rata salary of as little as £10,560 per annum, it says.
Salary arrangements vary according to local authority.
Ms McAnea said that it was not new for school support staff to carry out overtime.
"These results confirm our view that workforce reforms are slow to deliver tangible benefits to school support staff," she said.
Teachers are now entitled to spend 10% of their time out of the classroom - planning, preparation and assessment time (PPA).
Some unions had warned that schools which could not adequately fund PPA would rely on classroom assistants to take on more responsibility by covering teachers' PPA time.
The National Union of Teachers did not sign up to the PPA agreement. The National Association of Head Teachers did, but subsequently pulled out.
Schools' minister Jacqui Smith has said schools' funding settlements for 2005-06 meet the costs of PPA, and include a minimum funding guarantee of 5% for primary schools.
A spokesperson for the Department for Education and Skills said decisions on pay and contractual arrangements for school support staff were best taken at local level, as they can be more flexible and responsive to local needs.
"The National Agreement on raising standards and tackling workload makes clear that we expect support staff pay to reflect their level of training, skills and responsibilities."
"For 2004-05, we have included £279m to contribute to support staff salaries and £51m for their training and development in a new single grant for schools, the School Development Grant, worth £647m in total," he added.