Schools Leaders Scotland (SLS) wants firm details by mid-September at the latest of the back-up plan if Covid-19 leads to the cancellation of the 2021 exams.
SLS, which represents secondary school leaders, also wants clearer guidance on how teachers should gather evidence of students' progress this year.
In an interview with Tes Scotland, SLS general secretary Jim Thewliss said that this year's Higher students, for example, had already missed several weeks in school at the end of National 5 courses last year, and then several weeks at the start of Higher courses before the summer.
He added that, at the start of the new school year, teachers lacked detail on how they should assess students and gather evidence to ensure fair grades from the Scottish Qualifications Authority (SQA).
The concerns come amid the continuing fallout from the 2020 exams debacle, which saw teachers' estimated grades eventually replacing those driven by a controversial algorithm – and which helped set in motion similar events in other parts of the UK.
Mr Thewliss today raised concerns about practical subjects in particular – such as PE, home economics and drama – where teachers were hampered by, for example, PE having to take students outdoors and practical cookery lessons not taking place in many schools at the moment.
He said schools had to be prepared for numerous eventualities, including the pupil absence rate "starting to creep up", and sudden Covid-related closures at a school, local authority or national level.
In the SLS submission to the SQA consultation on 2020-21 courses – to which more than 23,000 people responded – Mr Thewliss said: "We have concern over the level of future-proofing within these proposals. What plans are there in the event of another national or a local lockdown?"
He also said: "By the time these amendments feed into the system and even with a later exam diet at least five weeks of teaching time will already have been lost for next year’s exam.
"Furthermore the proposals do not take account of the fact that many pupils hadn’t actually finished last year’s course before lockdown. Significant catch-up and refresh time has been required to make up for both of these deficits in learning."
SLS would also have preferred to see "a greater emphasis on moderated judgements/internal assessment pieces, particularly for National 5", adding that if the SQA had "adopted that approach then it could also widen and promote the debate about the purpose of exams".
Mr Thewliss warned that the consultation's focus on changes to individual subjects presented "the inherent danger of inequity of impact on individual children given the selection of subjects studied".
The EIS union's Education Committee has also made a submission to the SQA consultation. EIS education convener Susan Quinn said: “Whilst some of the proposed changes are worthy of support the general feeling of the committee was that at best they amounted to marginal tinkering, some of which seemed more designed to make things easier for the SQA rather than relieving pressure on schools and students.
"Working towards a normal diet of exams seems hopelessly optimistic on the part of both the Scottish government and the SQA, given even the current levels of disruption being faced in schools, let alone the threat of a second wave of the virus.”
“In both the short and the longer term, we need to have less reliance on high-stakes examinations to accredit student performance.”
The first meeting of the EIS Education Committee in the new school year heard from teachers across Scotland angered by the SQA results fiasco.
The union said they were "scathing" about the SQA, "in particular its failure to engage schools in professional dialogue before upholding results derived from its now-infamous algorithm".
Ms Quinn said: “The EIS Education Committee is not normally a place where serious anger is expressed, but there was certainly a considerable amount of ire at this week’s meeting over this year’s SQA qualifications experience.
"Teachers were outraged that professional judgements on student achievement seemed liable to be overruled by a computer in the SQA moderation process."
Ms Quinn added: “It should not have taken a massive public and political backlash to force a more sensible and compassionate model for recognising student achievements this year. Teachers know their students well and acted with commitment and professionalism throughout this year’s qualifications process – despite the additional workload pressures that this presented.
"The SQA has lost a lot of teacher goodwill in how it has handled matters."
An SQA spokesperson said: "More than 23,000 people, including 5,000 learners, parents and carers, provided responses to our two consultations [on 2020-21 courses]. We have also engaged directly with learners.
"Thanks to all practitioners, young people, parents, carers and other organisations who replied.
"The feedback is now being analysed and we will publish the outcomes as soon as possible."