A Curriculum for Excellence could go "very badly wrong", David Cameron warned secondary heads last week.
A common language existed - everyone could recite the four capacities - but there was no common understanding of what they actually meant, he claimed.
Schools were waiting for clarification to come down from on high - from civil servants and politicians. But they needed to start taking matters into their own hands, Mr Cameron told delegates at the annual conference of the Headteachers' Association of Scotland in Cumbernauld last week.
"People should not be waiting for tablets of stone to descend from somewhere to tell them what A Curriculum for Excellence is and means," he said. "We need to start answering that question. Work needs to be done quickly, but that does not preclude the possibility of moving forward in areas where there is clarity.
"This is our opportunity to control development and do what we want to do. So do we want to show courage, or caution? Do we want to be leaders of this, or do we want to be victims? For me, there is one choice: let's be leaders, explore and experiment, and have a curriculum for the 21st century."
But standing in the way of ACfE was pressure to produce attainment results, headteachers claimed. Current examinations and the restrictions they placed made one of the central aims of the new curriculum - more freedom for teachers - difficult to achieve.
Arthur Jones, headteacher of Largs Academy, said: "Unless SQA tell me they are scrapping Standard grades and Highers, I am expecting teachers to be free and liberal and concentrate on attainment."
This view was echoed by Ken Cunningham, headteacher of Hillhead High in Glasgow, who said: "There is a tension between the exciting teaching we want in school and how we measure attainment."
Mr Cameron argued that a broader curriculum and attainment were not exclusive. "If we offer young people an experience that is engaging, motivating and stimulating, then that will translate into academic success," he said.
"We need to have courage. If you engage youngsters through a broader curriculum, it does not mean they can't get a qualification in English, maths or science."
Brian Cooklin, incoming president of HAS and head of Stonelaw High in South Lanarkshire, likened getting to grips with ACfE to yogic flying: "You hope that something comes to you through the ether." He suggested that, in secondary schools, there was "nothing happening that's not A Curriculum for Excellence", adding: "It's a way to earn Brownie points."
Mr Cooklin quipped: "Develop a group that supervises the toilets - that's confident individuals."
The open season on the new curriculum moved on this week to the annual conference of the education directorate in Aviemore - with a warning that it could lose their support.
Bruce Robertson, outgoing president of the Association of Directors of Education in Scotland, told his colleagues: "A Curriculum for Excellence was an area that I recall outlining with some frustration over a year ago. What real progress have we made here nationally?
"Some schools and some authorities are taking the bull by the horns but, as 2008 looms and curriculum plans are being made for the next session, we would all have expected to have been much further advanced.
"The initiative started off life with the full backing of the education community in Scotland, and I suspect this year will be a make or break year for it."