Schools are threatening legal action against the Government over spending cuts that could rob up to 50,000 teenagers of their sixth-form places in September.
Headteachers are warning they will have to drop A-level subjects such as French and German and further maths if they do not get the cash reinstated.
Rob Bevan, head of the 1,000-pupil Southend High School for Boys in Essex, accused the Government of "incompetence" and bordering between being "disingenuous and dishonest" over the preparation of sixth-form budgets.
He faces the loss of £80,000 (6 per cent) from his sixth-form budget – effectively meaning at least 10 pupils will not be funded from this September.
A similar picture emerges in other secondary schools, sixth-forms and further education colleges throughout the country – with the worst affected facing losses of more than £160,000 (the equivalent of 40 pupils' places). Overall, the budget cut has been estimated at around £200m.
Mr Bevan, whose school is one of the 164 remaining state grammar schools in the UK, said: "We wouldn't change our offer of places for September because we're committed to the students who have signed up for education.
"But in the long run, if this money is not found we'd have to look at subjects taken by a minority of students. These could be French, German, further maths, design and technology and music technology."
Mr Bevan, an executive member of the Association of Teachers and Lecturers, said schools were considering seeking a judicial review of the decision by the Learning and Skills Council, the body responsible for funding post-16 education. It would be a "class action", with as many schools as possible signing up.
Heads base their case on the fact that they received letters at the beginning of March from the LSC detailing what was then described as their "final budget notification". On 31 March, a day before the new budget came into forced, they received a new email outlining the cuts.
The LSC argued that it had understimated the number of youngsters planning to stay on in education as a result of the recession.
"It was bizarre," said Mr Bevan. "They actually mentioned the cut (in pupil funding) in a footnote on the 13th page (of the email attachment). I would go so far as to say it is dishonest – bordering between disingenuous and dishonest."
Michael Gove, the Tories' schools spokesman, said: "Anyone would say it was catatrophic incompetence to say something was final when it was provisional." He said Schools Secretary Ed Balls could not escape blame, because his department had an observer on the LSC.
Teachers' leaders have pointed out the shortfall in funding threatens at least three government objectives: raising the education leaving age to 18; increasing the number of youngsters opting for higher education to fifty per cent; and persuading more 14 to 19-year-olds to study its flagship new diplomas.
When he addressed the conference on Monday, Mr Balls said: "The numbers of young people wanting to stay on in school are considerably higher than last year." He hinted that extra money would arrive as a result of the Budget next week. "Give us until the end of April to try and sort out the funding," he said.
Earlier this year the LSC's chief executive Mark Haysom resigned after it emerged nearly 80 colleges would not have enough money to carry out promised building works.