Three-quarters of state-run secondary schools in Calderdale are spending more than they receive in funding, new figures reveal.
The National Education Union has warned that insufficient funding means schools across the country are struggling to make ends meet.
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Department for Education data shows three of the four local authority-run secondary schools in Calderdale (75 per cent) finished the last financial year in deficit.
This means their budget was not enough to cover all their costs during 2018-19.
More than 28 per cent of secondary schools across England are now spending more than their budget, up from 11 per cent five years ago.
The figures exclude academies, which are government-funded but are not overseen by the local authority.
The Department for Education says the proportion of schools in deficit last year dropped for the first time in recent years, with 53 fewer schools now in the red.
However, more than three-quarters of these schools were converted to academies during the financial year, and are therefore no longer included in the statistics.
Kevin Courtney, joint general secretary of the NEU, said it was "disturbing" so many schools are in deteriorating financial positions, given many had already made deep cuts.
He said: "Throughout the election we warned that, under Conservative plans, schools will have £2 billion less spending power in 2020-21 than they did in 2015-16.
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"The stark reality of this is that even with additional money coming in from April 2020, the vast majority of schools will still be struggling to live within their means.
"Government funding has simply not kept pace with inflation, nor the increase in pupil numbers, so what they now offer is completely insufficient to lift all schools out of financial jeopardy."
In Calderdale, the proportion of secondary schools in deficit has risen significantly from 2013-14, when it stood at 29 per cent.
They overspent by a combined total of £1.2 million over 2018-19 – an average of £383,676 each.
Overall, six per cent of all council-run schools – including nurseries, primary schools and special schools – in the area are now in deficit.
Natalie Perera, executive director and head of research at the think tank Education Policy Institute, said school balances were a key indicator of financial health – and that the figures were "not particularly positive".
She added: "The Government's new funding boost for schools may improve the picture in the long-term – but it's important to note that this additional investment merely returns school funding to levels seen in 2009-10.
"In the more immediate term, many schools in England will continue to feel a squeeze on resources."
A Department for Education spokeswoman said: "This Government has announced the biggest funding boost for schools in a decade, giving every school more money for every child.
"This means that every school in the country can see per pupil funding rise in line with inflation next year, with all secondary schools receiving a minimum of £5,000 per pupil.”