The latest assessment guidance for the new curriculum, issued last week, has caused confusion and been criticised for failing to spell out how and when pupils in S4-6 will sit exams.
A former leader of Scotland's secondary heads said the plans were like "swimming through semolina".
Larry Flanagan, education convener of the Educational Institute of 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, including the possibility of early presentation for Highers in S4, which he feared could affect pupils who moved schools frequently. EB.