Teachers across England are "solidly" supporting a 24-hour strike which has closed schools and disrupted lessons.
Members of the National Union of Teachers (NUT) walked out in a long-running dispute over school funding, pay and conditions and were joining rallies and marches.
The union said its members had shown strong support for the strike, which it added was being backed by many parents.
"Most schools where action is being taken are affected by closures or reduced subjects," said an NUT spokesman.
The NUT claims funding to schools is being cut, leading to increased workloads for teachers and bigger class sizes.
The union has written to Education Secretary Nicky Morgan calling for "meaningful" discussions to tackle issues it said were negatively affecting education.
Acting general secretary Kevin Courtney said: "The NUT is aware that strike action can be disruptive to parents and carers and for that we wholeheartedly apologise. Equally, teachers do not take strike action lightly. The problems facing education, however, are too great to be ignored and we know many parents share our concerns.
"The strike is about the underfunding of our schools and the negative impact it is having on children's education and teachers' terms and conditions.
"Schools are facing the worst cuts in funding since the 1970s. The decisions which head teachers have to make are damaging to our children and young people's education. Class sizes going up, school trips reduced, materials and resources reduced, and subjects - particularly in the arts - are being removed from the curriculum. Teaching posts are being cut or not filled when staff leave. All of this just to balance the books.
"No parent wants this for their children. No teacher wants this for their school or pupils. With political parties in turmoil since the EU referendum, it is imperative that education is put to the forefront of every election campaign. The problems schools face need addressing immediately. We must not let the education of the next generation be sidelined."
Ms Morgan has denied the union's claims that funding is being cut and had accused the union of organising a "political" strike.
Green Party leader Natalie Bennett offered her party's support for the strike, saying: "Teachers are striking in defence of the education their pupils, and future pupils, receive.
"Schools that I visit up and down the country tell me they are struggling to maintain the quality of education in the face of slashed funding. Some are being forced into making teachers and support staff redundant, which means a poorer education for pupils. Others are resisting that at the cost of spending on building maintenance, spending that can't be postponed indefinitely."