Thousands of pupils in England are seeing school closures or disruption as the National Union of Teachers stages a one-day strike over funding.
The government said figures so far suggested one in two schools was unaffected by the action.
The NUT is organising regional marches and rallies in support of the strike, which is also about pay and workloads.
Education Secretary Nicky Morgan said the strike was “unnecessary” and “harmful”.
The most recent government figures suggested that of England’s 21,957 publicly-funded schools, 52.7% were unaffected by the strike, 17.4% experienced some disruption, 10.2% were closed, while officials were unable to contact the remaining 19.7%.
Union officials claimed teachers across England were “solidly” supporting the 24-hour strike with hundreds of schools across the country affected, either closing completely, or teaching “reduced subjects”.
- In Liverpool and Cheshire, more than 130 schools are closed and around 240 are partially closed
- Across Greater Manchester 270 schools are closed or have reduced staffing
- In Oxfordshire, 38 schools are closed or partially closed
- In Hampshire, Portsmouth, Southampton and the Isle of Wight, 81 schools are affected
- Dorset, Bournemouth and Poole have reported 44 schools affected by the strike
“We have seen a 10% loss of funding in the past year,” said Liam, a teacher at a secondary academy in the east London borough of Hackney.
“The government says it has ring-fenced funding to schools, but this is not actually happening.
“We are seeing increased class sizes and a decrease in opportunities for students.”
Asmah, a teacher at a north London primary, said they had just been informed of a huge budget cut for next year.
“The uncertainty is quite worrying,” she said.
“There have already been redundancies.”
Tom, another primary teacher, said heads were having to make “drastic staffing decisions” with cuts to “valuable aspects of children’s education” such as arts and music.
Workload was the biggest issue for Eleni.
“As a full-time teacher, it is impossible to get a work-life balance,” she said.
Molly and Rebecca, teachers at a south London primary, were worried about salary cuts if their school became an academy.
“Academies don’t have to stick to the national pay scale or pay teachers for marking and preparation time,” they said.
Academy teacher John said he and his colleagues were expected to provide revision classes on Saturdays in the run-up to exams, without any extra pay.
“We are just expected to come in at weekends and in the evenings.
“I think the worst thing is the impact on the kids of tired and ratty teachers.”
The union’s acting leader Kevin Courtney said school budgets were not keeping pace with rising costs.
Mr Courtney said “teachers do not take strike action lightly” and he “wholeheartedly apologised” for disruption to parents.
“The problems facing education, however, are too great to be ignored and we know many parents share our concerns,” he added.
He told BBC Radio 4’s Today programme: “Class sizes are going up. We are being told of schools where there will be classes of 35 in September. Art, dance and drama teachers are being made redundant or not being replaced when they leave; individual attention for children is going down.
“This is all happening because the government is not allowing school budgets to keep pace with inflation. They are freezing the cash per pupil they give to schools.”
The NUT points to an analysis by the Institute of Fiscal Studies which says spending per pupil is expected to fall by about 8% in real terms by 2020.
He said the union wanted to resolve the matter through talks but the education secretary “does not acknowledge the reality”.
Mrs Morgan criticised the strike, saying it would “harm children’s education”, inconvenience parents and “damage the profession’s reputation in the eyes of the public”.
She said the government had protected school funding “when other areas of spending are having to be reduced”.
“Spending on education is the highest it has ever been this year at £40bn. It has gone up £4bn since 2011-12,” she told the BBC.
She said the way to resolve the issue was via talks rather than taking strike action.Labour’s shadow education secretary, Angela Rayner, also called for talks “to thrash out a better deal for the next generation, which is what every parent wants”.
“This Tory government is letting down parents, pupils and teachers.
“On their watch, class sizes have soared and the number of unqualified teachers in our classrooms is up,” said Ms Rayner.