The plea was prompted by a speech to the School Leaders Scotland annual conference last week by minister for learning Alasdair Allan, when he appeared to close off the option of pupils making their subject choices in S2.
"It is clear that, to deliver the full range of young people's entitlements, options for further study should not be prematurely cut off before the end of S3," said Dr Allan.
"This means, and Education Scotland has been clear on this point, that arrangements for choices in S1 to S3 should not narrow options for the senior phase, and should not involve traditional subject choice for all young people in a cohort at or before the end of S2."
SLS general secretary Ken Cunningham told TESS he was delighted to hear the minister elsewhere in his speech reaffirm the primacy of appropriate courses for youngsters that led to open pathways for future career advancement.
But he added: "Where we have a problem is where it becomes rigid in the interpretation of that, so that choices are made at the end of S3 only."
Keir Bloomer, one of the original architects of Curriculum for Excellence, recently warned that pupils risked narrowing their subject choice too early under certain structures. Others in the sector have cautioned privately that forcing pupils to do only five subjects in S4, for example, could make some lower-uptake subjects unviable and lead to workforce planning issues.
Mr Cunningham said: "We are wary that a rigidity in approach will cause problems down the line in terms of subject choice. It could be that the intention of broadening out the curriculum ends up doing the opposite."