School support staff including teaching assistants, technicians and caterers are "buckling" under increasing workloads caused by cuts and restructuring, according to a new report.
Research by the Unison union found that most staff said cutbacks in their school are having a "noticeable" effect.
A survey of more than 12,100 school employees across the UK, including office staff, found that seven out of 10 are carrying out duties they say should be performed by someone at a higher level.
Stress levels are "rocketing", with one in five having to take time off sick because of "intolerable" workloads, said Unison.
The union's head of education, Jon Richards, said: "School support staff who haven't already lost their jobs are buckling under intolerable workloads and mounting stress levels.
"They play a vital role in keeping children safe and schools running smoothly; they shouldn't be seen as surplus to requirements when money is tight."
A Department for Education spokesperson said: "Teaching assistants are an integral part of the school workforce, supporting the 450,000 teachers in our classrooms.
"In fact, we have seen a 50,000 increase in teaching assistants since 2010. We are working closely with the profession, unions and Ofsted to reduce paperwork and bureaucracy for all staff, including teaching assistants, and have recently published a workload reduction toolkit which should benefit all staff in schools.
"There is also more money going into schools than ever before, with funding rising to £43.5 billion by 2020 – 50 per cent more in real terms per pupil than in 2000 and the education secretary has set out his determination to work with the sector to bear down on cost pressures."