School budgets at “breaking point” are forcing headteachers to take “drastic measures”, a union leader has warned.

Heads of primary and secondary schools across West Sussex sent a letter to parents last week revealing funding cuts may result in a four-day week.

The county is one of the lowest funded in England with, according to local campaign group Worth Less?, pupils receiving £44 million less than the national average this year.

Peter Woodman, head of the Weald school in Billingshurst and chair of the West Sussex Secondary Heads’ Executive, told the Mail on Sunday: “What is frightening is when you start saying ‘do we have enough money to open five days a week?’

“We’re not saying we’re going there yet. We’re saying on a range of things: absolutely everything needs to be explored.”

What is frightening is when you start saying ‘do we have enough money to open five days a week?

By law schools are required to educate pupils for 190 days in each school year, meaning they would need to be open for 48 weeks on a four-day timetable.

But Malcolm Trobe, interim general secretary of the Association of School and College Leaders, said it was “inevitable” that schools would have to make further cuts of a “similar nature”.

“The reality is that school budgets in many areas of England are at breaking point and there is no alternative but to consider drastic measures.”

It was also reported this week that Latymer grammar school, in north London, had asked parents for donations of up to £600 a year to meet a “very significant shortfall”.

Schools Week also found another grammar school – Tiffin, in Surrey – has previously asked for parent donations.

Robert McCartney, chairman of the National Grammar Schools Association, said there was an ongoing “bitter war” about funding in grammar schools, which felt the same budget pressures as other state schools.

Investigations by the British Humanist Association last year uncovered 100 state schools asking for financial help from parents.

Trobe said his union has pressed the government for “transitional relief” for schools in the lowest funded areas of England after the government delayed the introduction of a new national funding formula from 2017 to 2018.

The Department for Education said it had “protected the schools budget so that, as pupil numbers increase, so will the amount of money for our schools”.