Thousands of schools are expected to close next month when the leading union for school support staff plans to hold two days of strikes.
Unison, which represents 200,000 school staff such as classroom assistants and caretakers, was today due to confirm industrial action on July 16 and 17.
It could be joined by the 70,000 school-based members of the Unite union, who are still balloting over whether to take strike action over this year's 2.45 per cent pay offer.
Many headteachers are already assessing the health and safety risks of keeping schools open without support staff, who represent around half of the total school workforce.
Heads' associations have said many will have learnt lessons from the National Union of Teachers' strike in April, when 9,500 schools faced closure or part-closure.
John Dunford, general secretary of the Association of School and College Leaders, said the absence of a caretaker, catering staff and classroom assistants would pose "a major problem", but difficulties would be minimised because the strike was due to be held in the last week of term for many schools.
The level of disruption is expected to be very varied. In February, a local strike by Unison, Unite and the GMB union in Birmingham shut 168 of the city's 430 state schools. Some areas may have more support staff in the GMB, which is not taking part in the July strikes.
Schools with high numbers of special needs pupils are expected to be the worst affected.
In east London, where Unison is dominant, schools say they are already used to coping with action.
Tim Benson, head of the 900-pupil Nelson Primary in East Ham, said around half of his entire 100-strong staff could be on strike.
"We are an inclusive school with many with special needs, and most of our Unison workers are learning support assistants," he said. "What we usually do in these situations is work through to 1pm, then close."
Unison has complained that teaching assistants are being treated as "teachers on the cheap".
Christina McAnea, head of education at the union, said the average full-time teaching assistant earned just pound;11,000 a year.
"Local government workers are among the lowest paid and they feel the impact of rising food and fuel prices faster than others in the workforce," she said.
Join the debate at www.tes.co.ukstaffroom.