The Association of School and College Leaders says England's schools have had to make more than £1bn savings this year, rising to £3bn by 2020.
The government says school funding is at a record £40bn, with rises ahead.
Education Secretary Justine Greening will hear heads' cash grievances at Friday's ASCL conference in Birmingham.
She is due to address the union, which has published a survey of its members on the issue.
It suggests schools are finding it difficult to make savings without cutting provision and that things are predicted to get worse over the next two years.
Cost pressures are rising as greater pay, pension and national insurance costs are having to be covered from school budgets.
ASCL complains a new funding formula for schools has reduced the basic level of school funding going forwards by too much.
The meeting comes two days after requests for more money to spend on daily school costs were ignored by the chancellor in the Budget.
Philip Hammond however did pledge £500m for school buildings, mainly new free schools - some of which could be grammar schools.
One respondent said his school was moving to a "bare bones education", in which "the components that make education special and enjoyable are being eroded away".
Some 95% of the 1,054 heads, deputies and senior teachers responding to the survey said they had cut back on support services - including equipment and materials, as well as mental health and special needs support.
More than eight out of 10 said class sizes had increased - a claim strongly refuted by the Department for Education.
And more than two-thirds said they had cut back on activities like clubs and trips.
Just under three-quarters of respondents with GCSE-level classes said they had cut courses and just over three-quarters of heads with A-level students said they had also reduced subjects.
Foreign modern languages, music, arts and drama were among subjects removed at A-level.
Another said: "Through no fault of their own, students will have restricted subject choices, in larger class sizes with less pastoral support, whilst still being expected to perform at the highest of standards - nonsense!"
One head said his school may have to axe its sixth form provision for next year and another said his school was starting to "creak" with all staff working to full capacity.
Interim general secretary, Malcolm Trobe, said: "School leaders will do their utmost to protect provision, as they always do, but they cannot provide everything that is asked of them without the resources they need.
"Unless the government invests more in the education system, there will be a significant impact on the lives and life chances of young people."
A spokesman for the DfE said: "As this week's Budget demonstrates, the government is determined to ensure every child has access to a good school place and is given the opportunity to fulfil their potential.
"The government has protected the core schools budget in real terms since 2010, with school funding at its highest level on record at more than £40bn in 2016-17 - and that is set to rise, as pupil numbers rise over the next two years, to £42bn by 2019-20."

Chile beat Argentina on penalties last year to win a first major trophy.
The 100th anniversary of football's oldest international championship sees hosts USA drawn with Colombia, Costa Rica and Paraguay.
Uruguay, the tournament's most successful team, play Jamaica, Mexico and Venezuela, while 2007 winners Brazil face Ecuador, Haiti and Peru.
The 2016 Copa America has been expanded by four teams and is being staged outside the South American Football Confederation for the first time.
The tournament starts on 3 June when USA play Colombia in Santa Clara, California.
The final is on 26 June at MetLife Stadium in East Rutherford, New Jersey.
Head teachers say they are axing GCSE and A-level subjects, increasing class sizes and cutting support services as they struggle with school funding.
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Holders Chile have been drawn in the same group as Argentina, Bolivia and Panama at the 2016 Copa America.