Schools will have to fully implement the new curriculum by next year
Secondary schools are facing a crisis similar to the Scottish Qualifications Authority (SQA) exam crisis ten years ago, a headteachers' union has claimed.
Carole Ford, outgoing president of School Leaders Scotland (SLS), said the government's Curriculum for Excellence was unworkable in secondary schools.
She said she was raising the alarm now because there was still time to fix it.
But the government said SLS had been "fully involved" in the development of the curriculum.
The Curriculum for Excellence will be fully operational by August next year.
Ms Ford told BBC Scotland: "In the run up to the SQA exam crisis ten years ago it seemed to be the same story.
"Thousands were saying this isn't working but it was brushed aside. We're raising the alarm now because there's still time to fix it."
In 2000, thousands of students received unreliable exam results after management failings at the SQA. In a speech to the SLS, which represents the majority of secondary headteachers in Scotland, Ms Ford was due to say that the concerns of educational professionals had "fallen on deaf ears".
The new approach to the curriculum will affect pupils from their nursery years through to their secondary education.
The government has said that the change has come about because the world is changing.
It believes that because of new technologies the way lessons are taught has to change.
But Ms Ford has insisted that the way the new curriculum had been crafted without the rightful input of people who work in schools.
She said she had chosen the curriculum as the theme of her outgoing speech because of her "increasing concern that the opinion of professional educators in schools, and the evidence that educational research provides for us, do not carry sufficient weight in decision making circles".
Education Secretary Fiona Hyslop said the concerns were about qualifications that would not be introduced for a number of years.
She told BBC Radio Scotland's Good Morning Scotland programme: "Criticisms here have been about the detail of assessment which actually haven't been released.
"I think the importance and the role of headteachers developing to the stage to which we've got has been very important indeed."
'Support of headteachers'
Ms Hyslop said she has "stuck her neck out" for headteachers in the past.
She added: "I won't accept any proposals that don't have the support of headteachers and teachers and I recognise in the new assessment coming forward that there's been a misunderstanding I think by some of the headteachers."
Ms Ford, who will deliver a speech to her union's conference later, has three areas of serious concern which include:
• assessments - she said that although the aim of the new curriculum was to reduce time spent on assessment, in only one area was there a "diminution" in assessment
• measuring achievement - she said it was impossible to measure the curriculum's key objectives of citizenship, pupil confidence and pupil contribution
• literacy and numeracy - she said she had very serious concerns about assessment for literacy and numeracy because there was no formal measure until the 10th year of a child's education.
Ms Ford said: "We know that the early years of education are the most important in relation to both these skills, yet there is to be no formal mechanism for measuring progress, no standardised means of communicating progress to secondary schools at the transition stage, and no accountability during the first seven years of education."
The president's comments follow a warning from HM Inspectorate of Education that the Curriculum for Excellence could turn out to be superficial.
Scottish Labour said Ms Hyslop must engage with Ms Forde and explain why SLS representations to minister had apparently fallen on deaf ears.
The party's education spokesman Des McNulty said: "Teachers and headteachers feel they are not being supported - there is a lack continued professional development and their opinions are not being listened to.
"These latest concerns expressed by headteachers representatives come on top of a catalogue of failure by the SNP in education."