A shortage of teachers has left school support workers picking up the slack, a survey by a teaching union suggests.
And often, it appears, they do so without any extra pay.
A third of those polled by the Association of Teachers and Lecturers (ATL) said they work four or more extra hours each week, while nearly three quarters said they do not get paid for the extra work they do.
One teaching assistant in a primary in Warwickshire said: "I understand that budgets are tight in schools but that is no excuse for how support staff are treated.
"I cover teachers two days a week during which time I teach the class. The financial reward for doing this is barely noticeable in my wages."
More than 42% of the ATL members surveyed said that they cover classes, while 64% said they did not feel that the work they do as a cover supervisor is any different to that done by a supply teacher.
ATL general secretary Dr Mary Bousted said the government needed to address the workload of school staff.
"It is unacceptable that so many support staff are working longer hours than they are contracted for," she said.
A Department for Education spokesman acknowledged that "unnecessary workload" was a major frustration for teaching staff, but indicated the responsibility lay with the schools
"We trust heads, governors and academy trusts to plan their staffing," he said.
"Support staff are best used when they add value to what teachers do, not when they replace them."
The ATL survey polled 1,763 union members working as support staff in state-funded and academy schools in England, Wales, Northern Ireland and the Isle of Man in autumn 2015.