The latest assessment guidance for the new curriculum, issued last week, has left confusion in its wake. It has come under fire for failing to spell out how and when pupils in S4-6 will sit exams.
The plans were dismissed witheringly by a former leader of Scotland's secondary heads as "swimming through semolina".
One commentator said the problem was that "everyone uses the same words which mean something different - they all have their own variation of what it actually means".
Larry Flanagan, education convener of the Educational Institute for Scotland and a member of the management board in charge of A Curriculum for Excellence, insisted that S4 would become an exam-free year for most pupils.
But Ann Ballinger, general secretary of rival union, the Scottish Secondary Teachers' Association, said she expected pupils to be sitting external exams in S4, S5 and S6 in mixed-year groups and feared that multi-level teaching would become far more common than at present.
Brian Cooklin, past president of School Leaders Scotland, anticipated different models being adopted, which he feared could affect pupils who moved schools frequently. This would include the possibility of early presentation for Highers in S4.
But Mr Flanagan insisted that fourth-year students would sit no formal exams, apart from the new literacy and numeracy qualifications which will be based on external assessment of a portfolio of documents rather than a sit-down "test".
S4 pupils would, however, be able to gain the new National 4 qualifications, which are internally assessed and replace Standard grade General and Intermediate 1.
Those staying on to S5 would sit either National 5 exams (externally- assessed replacements for Standard grade Credit and Intermediate 2) or Highers, and be awarded the National 4 qualification en route.
Pupils should not be sitting National 5 exams in S4, Mr Flanagan insisted, because the system was predicated on National 5 qualifications and Highers being two-year courses. At some point in S5, the decision would be taken whether an individual pupil would go for National 5 or Higher, depending on his or her progress.
By giving pupils two years to study towards Highers, or the level below that (National 5), there would be more time for teaching and learning, he suggested: "Pupils will be studying more than the current five Highers in S5, so will have greater breadth. It will allow for pupils to do six Highers, a skills course and a community involvement course, so breadth will be supported."
The SSTA, however, said that most schools planned to treat S4-6 as one unit, timetabling pupils from all three year groups together, depending on the level of course they were sitting. It also expected pupils of different abilities to complete exam courses over different time periods and warned that, in small secondaries or in subjects taken by just a few pupils, teachers would have to teach multi-level courses - in some cases, pupils at National 4, National 5 and Higher standard all in the same class.
Ms Ballinger argued that the biggest flaw was that the most able pupils would move straight to Higher without having sat an external exam.
Judith Gillespie, development manager of the Scottish Parent Teacher Council, said the main problem was the premise that the new curriculum was based on acquiring skills, not content. While it might be possible to study English and, part of the way through the course, decide whether to sit a National 5 exam or a Higher, this was not an option in subjects such as science or maths because knowledge content had to be built up.
She also asks whether any meaningful qualifications will be available in school after S4 for youngsters who do not have the ability to sit National 5s or Highers and fears a return to something akin to the discredited Scotvec modules pre-Higher Still.
"This has been done entirely by the providers," she said. "There has been no serious input from the consumers. No one has been asking: `What happens to this child?'"
Education directors say they expect new vocational qualifications to become part of the qualifications framework, some of which will be certificated in S4, and say the picture is not yet complete. Flexibility is the name of the game, if every child's needs are to be met, they argue.
They acknowledge, however, that there is still work to be done on the senior phase. The emphasis on the acquisition of skills rather than content would allow a number of courses to be taught in multi-level classes, but directors acknowledge that this would not work for subjects based on "sequential" learning, like maths.
A Government spokeswoman said: "The new approaches will take place within a clear framework of assessment and qualifications that reflect the principles of ACfE. The information will be there to support professionals in making this change, with the new National Assessment Resource, the forthcoming assessment framework, and focused CPD."