School budgets are close to breaking point in England suggests research by a head teachers' union.
A survey of more than 1,000 schools by the National Association of Head Teachers (NAHT) shows the number in deficit has doubled since 2015.
And 71% of the heads polled were only able to balance their budgets by making cuts or dipping into reserves, said NAHT general secretary Russell Hobby.
The government said it had protected the core schools budget in real terms.
The NAHT's Breaking Point survey for 2016-17 shows:
- more than twice as many schools in deficit since 2015 - up from 8% to 18%
- 71% of heads balance budgets by making cuts or using reserves
- 72% fear budgets will be unsustainable by 2019
- 85% save money by spending less on new equipment
The greatest cost pressures on schools, according to heads, are:
- government changes which have passed the costs of employing staff on to schools
- the decline in local authority services and the abolition of a central government grant which allows councils to support schools
- supporting greater numbers of pupils with mental health issues.
According to the union, almost 80% of schools are providing support for children with mental health issues from general school budgets, "stepping in where cuts in health and social care funding have failed to meet the growing demand for support".
Mr Hobby said 98% of schools were losing funding "at a time when costs are rising and pupil numbers are growing".
"The government must take urgent action and commit to funding schools sufficiently in the next Budget. It is time to stop viewing education spending as a cost and to start seeing it as an investment in England's future, and in our children's."
Liam Collins head of Uplands Community College in Wadhurst, East Sussex, said budget pressures amounted to "a cut of 10 teachers, fewer clubs, no pastoral support, a narrowed curriculum, no counselling for students struggling with mental health issues, crumbling buildings, no IT upgrades, no new textbooks and no school planners.
"Eventually this will impact on student outcomes."
Labour said the NAHT's survey showed ministers "continue to hide their heads in the sand" on school funding.
Shadow education secretary Angela Rayner said: "Headteachers should not be faced with a decision of whether to cut school staff or invest in new equipment.
"The Tories have no plan to deal with falling budgets, chronic teacher shortages and not enough good school places - this is no way to run a schools system."
A Department for Education spokesman said that school funding "will be over £40bn in 2016-17 - its highest level on record".
The spokesman said the government's fairer funding proposals to end what ministers have termed "a postcode lottery" in school budgets would help.
"These proposals will not only see more than half of England's schools receive a cash boost in 2018-19 but will also give head teachers certainty over their future budgets, helping them make long-term plans and secure further efficiencies," said the spokesman.
"We recognise that schools are facing cost pressures, which is why we will continue to provide advice and support to help them use their funding in cost-effective ways, including improving the way they buy goods and services, so they get the best possible value for their pupils."