Chad Gething cannot help being jealous of teachers at his school, with their guaranteed non-contact time.

The pound;15,000-a-year learning support assistant at Mapledown special school in Barnet, north London, has frequently worked four or five hours a week of unpaid overtime during the past 12 months.

A survey by Unison, the publicsector union, shows Mr Gething is not unusual. It discovered that classroom assistants do more unpaid overtime than any other local government workers except the most senior council staff.

Seven out of 10 assistants work extra hours without pay and more than a fifth do at least four additional hours a week, a survey of 4,000 council workers found.

Unison accused schools of exploiting the goodwill of low-paid support staff, with more than half of assistants, nursery nurses and administration staff reporting increased workloads and stress.

Mr Gething said colleagues at other schools are put under pressure to put in extra hours before and after school. “You start off doing it as a one-off but it soon becomes expected. Headteachers will say, ‘We would pay you extra but there is no money in the budget’ and, ‘it is the children who will suffer if you don’t do it’.”

The survey, carried out by MORI for Unison showed 54 per cent of all local government workers do no unpaid overtime.

Full-time teaching assistants can take home as little as pound;10,500 a year. Many schools do not pay assistants during holidays.

More than 70 per cent of teaching and learning support assistants said they were dissatisfied with their pay. Mr Gething, 29, said two-thirds of his salary is spent on childcare costs.

While slightly more than a quarter of chief officers reported doing more than four hours’ unpaid overtime each week, only two out of five were unhappy with their pay. Chief officers in big authorities can earn up to pound;155,000 a year.

Christina McAnea, Unison’s national secretary for education staff, said:

“We have known for many years that school staff do a substantial amount of unpaid overtime. When you take into account that many of these staff work part-time and are low paid, their unpaid work constitutes a huge percentage of their working week and pay.

“The goodwill of support staff is being exploited by schools and local authorities who depend on their commitment to get the job done without paying them for the hours they actually work.”

Teaching assistants are taking on more prominent roles in schools as a result of the workforce agreement which allows classes to be taken by people who are not qualified teachers.

Employers have rejected the union’s demands for a national pay system and the union is now negotiating with ministers.

Despite the changes only a third of assistants have had their training needs assessed in the past 12 months, the survey showed.

More than half of caretakers and technicians are also unhappy with their pay.

* jon.slater@tes.co.uk