Headteachers were last night sceptical about claims by the Chancellor that the schools budget would continue to be ring-fenced.
They pointed out that the protection only applied to schooling up to the age of 16 and did not cover education for 16 to 19-year-olds.
As a result, school sixth-forms and colleges were already being "seriously squeezed" and being forced to cut courses.
Malcolm Trobe, deputy general secretary of the Association for School and College Leaders, said: "We welcome the confirmation in today's budget that school funding will be protected but this is somewhat misleading in that it only applies to education to age 16.
"Options for 16 to 19-year-olds are being seriously squeezed by funding cuts and we have already warned that many school sixth-forms and colleges will be forced to cut the number of courses on offer. In the long-run small sixth-forms could become unviable but even large sixth forms and colleges will face difficult decisions."
He added: "It is a fact that funding for 16-19 study will be significantly for lower for pre-16 and much lower than funding that succeeds it in higher education, The 16-19 funding dip is a growing and unwelcome anomaly. Given that the education participation age is rising to 18, this is a worrying trend."
Christine Blower, general secretary of the National Union of Teachers, added: "Schools and colleges have not been 'protected' as the Government claims but have suffered significant real term funding cuts."