(BBC) Many pupils in England are experiencing school closures or disruption as the National Union of Teachers stages a one-day strike over funding.
NUT acting leader Kevin Courtney said school budgets were not keeping pace with rising costs.
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”.
Nearly 25% of NUT members voted in the strike ballot – of those who voted, 91.7% supported strike action.
The unions claimed teachers across England were “solidly” supporting the 24-hour strike and were joining rallies and marches later in the day.
“Most schools where action is being taken are affected by closures or reduced subjects,” said an NUT spokesman.
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 plan, 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.