Funding pressures mean pupils at sixth-form colleges in England must choose from an increasingly narrow range of A-level subjects, a study has found.
The Sixth Form Colleges Association's annual survey suggests two-thirds of colleges have had to drop courses.
Over half (58%) have also reduced or removed extra-curricular activities such as music, drama and sport.
The government said it had protected the base rate of funding for all post-16 students until 2020.
There are more than 160,000 young people aged 16 to 18 currently enrolled at a sixth form college and the sector accounts for around 20% of the A-levels taken in England each year.
The SFCA sent its annual questionnaire to all 90 sixth form colleges in England in September, and 80 responded.
The findings indicate:
- Over a third of colleges (39%) have dropped courses in modern foreign languages, with A-levels in German, French and Italian being the main casualties
- 84% of colleges are teaching students in larger class sizes
- Nearly two-thirds of sixth form colleges (64%) say the amount of funding they will receive next year will not be sufficient to offer the support needed for disadvantaged students
- 90% of colleges are either extremely concerned or concerned about the financial health of their institution
The SFCA says the sector has suffered three funding cuts since 2011 and has also had to contend with rising costs in increased employer contributions to pensions and national insurance schemes.
"This year's funding impact survey suggests these two increases [pensions and national insurance] will cost the average sixth form college an additional £189,932 per annum," the report says.
It says the absence of a VAT refund scheme for sixth form colleges - which is available to school and academy sixth forms - leaves the average sixth form college with £385,914 less to spend on front-line education.
'It's an austerity year, I tell my staff'
Michael Hill, principal of Carmel Sixth Form College in St Helens, has had to increase teaching contact time for his staff by four hours a week, in an attempt to save money.
"Now that's hit the wider, extra-curricular activities because staff have got that much less time and energy," says Mr Hill.
"It tends to be the subject-based activities that suffer, like science club and debating society, which tend to be termly now rather than weekly."
Like many sixth form colleges, he has had to cut back on less well-subscribed courses.
"We've had to lose subjects - German and dance - just because the numbers aren't viable. Below 15 or 16 students and it's too difficult to run those courses."
Mr Hill also has less money to spend on equipment and materials - and a £100k photocopying bill will have to be reduced.
"The new specification A-levels means staff have wanted new textbooks and materials, but we've had to be quite stingy about what we've allowed."
But with many of his students coming from less well-off homes, Mr Hill is reluctant to increase their costs.
"I don't think we feel we can start charging students for materials for their courses - it would be quite difficult for a lot of our students to buy extras for art and sport and so on."
The SFCA report expresses concern that non-qualification and extra-curricular activities are being reduced or removed.
It says: "Enrichment activities such as educational visits, sport and the Duke of Edinburgh award play a vital role in developing the skills that are valued by universities and employers and help sixth form students to become engaged and active citizens."
SFCA chief executive Bill Watkin said the report should "act as a wake-up call" to the government.
"The message from the most effective and efficient providers of sixth form education is clear - more investment from government is essential if sixth form colleges are to continue providing young people with the high quality education they need to progress to higher education and employment.
"A review of sixth form funding is urgently required to ensure it is linked to the realistic costs of delivering a rounded, high-quality curriculum.
"Failure to do this risks turning sixth form education into a narrow and part-time experience. That would be bad for students, bad for society and bad for the economy".
A spokesman for the Department for Education said: "Every young person should have access to an excellent education and we have protected the base rate of funding for all post-16 students until 2020 to ensure that happens.
"We've also ended the unfair discrimination between colleges and school sixth forms and we now ensure funding is based on student numbers rather than discriminating between qualifications.
"On top of this we are providing more than half a billion pounds this year alone to help post-16 institutions support students from disadvantaged backgrounds, or with low prior attainment."