Funding pressures mean many sixth forms and colleges in England are cutting back on languages and sciences, the Sixth Form Colleges Association warns.
Schools and colleges have also cut back on services, including mental health support, the association says.
It is calling on the government to increase the rate of funding for students aged 16 to 19.
The government says it is looking at the needs of all colleges in the run-up to the next spending review.
The Sixth Form Colleges Association (SFCA) approached school and colleges in January and had 271 responses to its survey.
The results suggest just over half of school sixth forms and colleges have dropped courses in German, Spanish and French and more than a third have dropped Stem (science, technology, engineering and maths) courses.
Four in five head teachers and college principals said they had increased class sizes and more than two-thirds had moved from offering four subjects at A-level as standard to offering three subjects.
Three-quarters said the level of funding they had been allocated for the next financial year would not be enough to provide students with personal support, such as careers advice.
A similar number of heads also said they did not think they could adequately support disadvantaged students.
One principal at a further education college that supports a lot of disadvantaged young people said: "We cannot provide the extra wraparound activities that help young people make the most of their lives."
Pensions and national insurance
The SFCA says the funding rate of £4,000 per student for 16- to 19-year-olds has been frozen since 2013.
And it warns that the rate does not take into account the rising cost pressures in schools and colleges, such as increases in contributions to the teachers' pension scheme and national insurance.
SFCA chief executive Bill Watkin said: "If we are to keep key subjects on the timetable, offer a wide range of extracurricular experiences, and provide the essential support activities that our young people need and deserve, the government must raise the rate to at least £4,760 per student, per year."
A recent report by the Institute for Fiscal Studies said funding for 16- to 19-years olds had been cut more sharply than funding for schools, pre-school or higher education.
It found that funding per student in school sixth forms had fallen by 21% since its peak in 2010-11 and by 8% in FE colleges.
A spokeswoman for the Department for Education said: "Our further education and sixth-form colleges have a vital role to play in making sure people have the skills they need to get on in life.
"That is why we have protected the base rate of funding for 16- to 19-year-olds until 2020.
"We will also be providing £500m every year from 2020 to support the delivery of the new gold standard T-levels - which some sixth-form colleges will be offering.
"However, we recognise that the financial position for sixth-form colleges is challenging and are looking carefully at the needs of all colleges in the run-up to the next spending review."