School office staff in England and Wales are working the equivalent of £1m a year in unpaid overtime, according to one of the main unions.
The GMB says pay does not reflect increased duties
The GMB reckons administrative staff put in 97,000 hours extra each week.
It said two-thirds of the 57,000 staff were regularly working above their contractual obligations.
A GMB survey suggested one of the main reasons was the extra duties they had been given as a result of the agreement to reduce teachers' workloads.
The GMB's national secretary, Brian Strutton, said school admin staff were dedicated people.
"There is no doubt their goodwill is being taken for granted, especially when we hear the government saying it wants to increase the hours that schools are open.
"The sad fact is that, for all the talk of reform and investment, Muggins is alive and well and being relied upon to keep our schools running."
Their "excessive workloads" resulted from responsibilities for data being handed down from local authorities to school level.
"They are often the too-tolerant victims of inadequate staffing levels and school funding difficulties.
"They have had to take on an array of extra tasks as a result of the transfer of administrative and clerical duties previously undertaken by teachers as a result of the teachers' contract change."
But pay levels had failed to keep pace with the range of skills they now needed.
The typical salary was about £11,200 - less than half the national average wage.
"Low pay and excessive workloads are combining to sap morale and wreck work-life balance among a workforce which is pivotal to the smooth running of our schools," Mr Strutton said.
The union called on government, local authorities and schools to review the pay and grading of school support workers and to increase staffing levels so no-one had to work excessive overtime.
A spokesman said the GMB had regularly meetings with ministers and officials at the Department for Education and Skills about the practical implementation of the workload agreement and was keeping it under review.
The government says there are more than 90,000 more support staff in England than in 1997, and the number of teaching assistants has doubled to 122,000.
A spokesperson for the Department for Education and Skills in England said: "We have worked with the school workforce unions to dramatically improve the recognition, status and career opportunities for support staff in schools.
"Support staff are playing a valuable role in raising standards.
"Pay and contractual arrangements for support staff are determined at local level to suit local circumstances.
"However, the national agreement recognises that support staff should receive remuneration that reflects their level of training, skills and responsibilities."