Hundreds of pupils, teachers and parents sang “someone has to save our schools”, to the tune of Message in a Bottle outside Downing Street, amid warnings from campaigners that 250 schools plan to end the day at lunchtime on Fridays from September in an effort to balance the books.

Campaigners claimed the UK government was abdicating its responsibility to properly educate children after sustained, multibillion pound cuts. The sombre crowd marched from Parliament Square down Whitehall to the prime minister’s residence chanting “No ifs, no buts, no education cuts”, while banners saying “92 hours lost per year, per child” and “Give us 5” were held aloft.

Children and parents demonstrate in Westminster. Photograph: Martin Godwin/The Guardian

At one point, the Labour MP Jess Phillips’s 10-year old son, Danny, went through the gate to Downing Street with his mother and sat on the doorstep to work on his homework.

“I’m not just a policymaker, I am also a person and a parent, and when it’s people like me who are being affected by bad government policy, it’s much harder for them to ignore because I get to stand in front of them every day,” Phillips told the Guardian on Parliament Square. “The reality is that this is affecting people all across the country.”

“It is a basic fundamental principle that schools should be able to stay open five days a week ... Fifty schools have already had to make this decision.”

Asked whether missing lessons to protest against cuts to the school day was paradoxical, the MP for Birmingham Yardley said: “This is part of citizenship education, frankly. The children are learning that we don’t just let people with power walk all over us.”

In an earlier interview with Sky, Phillips criticised how class sizes at King’s Heath primary – her son’s state school in the West Midlands – were rising while the number of teaching staff declined, after the loss of about £200 per pupil in funding in recent years.

Kate Taylor, a teacher and campaigner from Save Our Schools, who organised the protest said: “Schools across the country are being starved of money. Real-terms funding has been reduced by 8% since 2010. Schools have cut absolutely everything they can. They’ve got nothing left to cut.

“They’ve cut staff, teaching assistants, they’re working in crumbling buildings, and for some schools the last thing they have left to cut is the length of the school day.”

Many of the protesters in Westminster were from King’s Heath primary, and seemed astonished at why the school week was about to be shortened. “The people who are making the cuts got five full days, why shouldn’t we?” one asked.

A governor stood nearby warned that places at the school for children with special educational needs such as autism, cerebral palsy and mobility issues were also under threat.

Jess Phillips with Danny (in orange) and other schoolchildren at the demonstration outside Downing Street. Photograph: Martin Godwin/The Guardian

“We cannot imagine implementing these horrific cuts,” the governor said on condition of anonymity. “But we may have to do it, even though it’s cheaper there than at special schools. There are no other places locally for children with severe needs.”

Ian Vogel, a sixth-form teacher and the father of a six-year-old at the school, said the “constant denials” that the education sector was being underfunded made parents furious.

He said: “The government keeps saying there is more money than there ever has been. But there’s more students in education than there ever has been, and the costs are higher than ever. [Boris Johnson and Jeremy Hunt] are talking about giving the richest people tax cuts, when actually we need them to pay more taxes to fund education.”

In a statement, Save Our Schools said figures from the Institute for Fiscal Studies showed funding per pupil had fallen by 8% between 2010 and 2018, and that £3.8bn was required to reverse school cuts, plus an additional £1.1bn a year until 2023.

“There are 66,000 more children in state schools this year than last, but 10,800 fewer staff, including 5,000 fewer teachers and 2,500 fewer teaching assistants,” it said, adding that a recent survey by the National Governors Association showed nearly a third of schools nationwide were already in deficit.