The figure was revealed on the eve of a demonstration in Westminster by parents and pupils protesting about a crisis in education funding, which means a growing number of children are being sent home at lunchtime every Friday so schools can save money.

Organised by Labour MP Jess Phillips, whose son’s Birmingham primary is among those affected, the march on Friday afternoon will be attended by protesters from Birmingham, Brighton, Hove, Hitchin, Wiltshire, Stockport, Hastings and Leicester.

To drive home the point, the Birmingham Yardley MP will deposit her 10-year-old son Danny and his best friend Morris on the front step of 10 Downing Street, where they will do their schoolwork, as a reminder of the government’s responsibility to care for and educate the nation’s children on a Friday afternoon.

“The whole thing is quite exciting for him,” said Phillips. “He wants to stick up for his school. It’s a brilliant school.”

According to Phillips, 26 schools in Birmingham including her son’s, Kings Heath primary, are either already on a four and a half-day week or are about to introduce it. Not only are pupils losing out on vital hours in school, parents are left scrambling to rearrange work or find childcare and school staff are losing jobs or pay.

“This is not just a Birmingham issue, which is what the government wants to paint it as,” she said. “It’s a problem in Stockport, Oxford, Cambridgeshire, Berkshire – there was one in Theresa May’s [Maidenhead] constituency – Bournemouth, London, Leicester, Sandwell.

“[The education system] is crumbling and nobody cares. The Department for Education just repeatedly wants to blame headteachers, as if the headteacher at my son’s school does not know how to manage money.”

Phillips said it was the responsibility of the government to make sure that every child is in school five days a week. “There are certain fundamentals in public sector services. Our children get free education five days a week. If you break your leg you can have a cast put on it. There are these fundamental principles that we expect from the state. Currently the state cannot deliver it.”

The campaign group Save our Schools (SOS) says children on four and a half-day weeks will lose 20 days of education over the course of a school year. They also point out the “hypocrisy” of government policies which leave schools with little choice but to close early, when parents face heavy fines for taking their child out of school for unauthorised absences.

“Every day, children are taught in crumbling buildings; subjects such as art and music are disappearing from the curriculum; pupils with special education needs are losing vital support and dedicated teaching staff are losing their jobs,” said SOS campaigner Kate Taylor.

“Now schools are being forced to reduce the length of the school week. Parents, teachers and pupils are living with the effects of a government that is not interested in investing in their education.

“If we were to remove our children for one Friday afternoon, let alone every Friday afternoon, we would be committing a criminal offence. It’s quite simple: we want our children to be in school receiving the education they deserve.”

Asked for comment on the protest, the DfE said flexibility over the length of the school week is not new and that schools have long had the choice to structure the school week as they choose. The department also pointed out that Birmingham’s funding was above average and should cover a full week of schooling for each child.