Headteachers are warning that severe budget cuts in schools are leading to courses being cut and class sizes increased.
Design and technology, languages and arts subjects are among those being dropped from GCSE and A-level courses, according to a new report.
The Association of School and College Leaders (ASCL) say that the budget cuts are having an impact on every area of school life.
Interim general secretary Malcolm Trobe said school leaders are being forced to make "impossible choices".
The Government has argued that school funding is at its highest ever level.
Despite this 72% of schools teaching 14-16-year-olds said GCSE or vocational courses have had to be axed in the past 12 months, while 79% of schools teaching sixth-formers said courses have been cut.
The report found that the most common GCSE subject to be removed was design and technology, followed by performing arts courses, music, German, art and design options.
At A-level, the most common course to be scrapped was again design and technology, followed by music and German.
Of those surveyed, 82% said they have had to increase class sizes.
On average, school leaders - including headteachers, deputies and other senior staff - said their largest class size is 33.
One school leader said there is a growing problem of increasing number of students being forced to study in "badly maintained and leaking buildings" as well as insufficient equipment.
"They are using out-of-date IT equipment which frustrates them as they tend to have the latest computers and tablets at home. Text books are becoming increasingly dog-eared", the school leader told the union.
The vast majority of schools surveyed, 95% - said that their school has cut back on support services, including equipment such as books, special needs support, IT and mental health support.
Over two-thirds (68%) said they have had to scrap activities such as trips, visits and clubs.
Mr Trobe said: "The survey shows the impossible choices school leaders are having to make. Reduced budgets means fewer staff and, with fewer staff, class sizes have to increase.
He added: "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 recent report by the National Audit Office has warned that schools will have to save £3 billion by 2019/20.