A complete shutdown of every state primary and secondary school in several areas of the UK is expected on Wednesday.
Two authorities – Blaenau Gwent and Newport in Wales have indicated every school will close. A third local authority, Sunderland, says every school will be disrupted with 89 closing and four facing partial closure.
A survey of local authorities by The Independent revealed support for the strike appeared harder in the country's industrial heartlands.
Only two schools in North Tyneside have been confirmed as open and just five in South Tyneside. In Durham, 168 out of 277 primary schools will definitely close. By contrast, only 44 of the 313 primary schools in Devon have said they will be closed.
Overall, from the 44 authorities who responded to our survey, a total of 2,421 schools declared they would either be fully or partially closed. If this picture was mirrored across the country, it would mean around 10,000 schools – mainly primaries – would close.
But the final figure is likely to be much higher. Some union leaders estimate that around 20,000 of the 26,000 schools in England and Wales will close.
A total of four teachers' unions and one headteachers' organisation are striking. The only union not to strike is Voice, whose members vow never to take strike action. It says it is receiving 100 applications for membership a day.
Many of those striking on Wednesday will be walking out of the classroom for the first time. John Paul, a primary school teacher for 33 years, said: "I'm hurt and offended by the injustice of what is being proposed. I didn't come into teaching to make a fortune – that's not what happens. I knew there would be no bonuses, no overtime but I did expect a decent pension."
Government concessions to those in their last 10 years of teaching would protect his pension, he admitted. But he said he had already lost hundreds of pounds as a result of basing the final figure on CPI rather than the RPI.
Support for the strike comes today from Education International, which represents 10 million teachers world-wide. It is publishing research showing £25bn was lost to tax avoidance in the UK last year –£18.5bn of which was lost to tax havens through avoidance by individuals and corporations. "It is quite clear from our report that billions of pounds are being held back from funding essential public services by corporate tax dodging," said general secretary Fred Van Leeuwn.
"It is absolutely wrong for the UK government to accuse teachers and other public service unions of being irresponsible and ignoring the financial crisis," he said.
Some parents groups condemned the move to take strike action. Margaret Morrissey, of Parents Outloud, said: "Teachers who strike – we expected better from you. You who lead our children you make no one feel proud and many feel ashamed. To the thousands of teachers who are working and not striking, thank you so much."
Is it right to strike? Teachers have their say
Yes: Julia Neal, a teacher at Torquay Girls' Grammar School, whose school will shut for the day
"I've been in teaching since 1978 and I've never taken industrial action, yet I'll have done it twice in six months [the Association of Teachers and Lecturers staged a one-day strike over the same issue in June].
I feel so strongly that teachers' pensions are being used to pay for other mistakes because the Government thinks it's an easy way out.
One of the things I really resent is for teachers to have to work until they're 68. We're already facing a two-year pay freeze and higher bills.
It's ludicrous for the Government to say we're acting prematurely. Our union has been trying to negotiate for months. We have to do this to safeguard the future of the profession."
No: Simon Smith, 51, is a teacher at Sweyne Park School in Rayleigh, Essex
The school will be open and those staff who aren't on strike will be expected to be in. It is a decision that I made many, many years ago that I could never reconcile in my own mind that striking was a successful way forward. That level of confrontation just isn't the way to achieve anything. Negotiations are the way forward.
Wednesday will be a normal day for me. I have two classes that won't be there but I shall utilise the time to catch up on marking and preparation. At our school people are committed to the school and the head is very fair. It's like we're punishing the school when we're taking action when the employer is Essex County Council.