Concerns have been raised by teachers that the new Nationals have created a "two-tier system" that risks saddling young people with qualifications viewed as worthless.
Learning minister Alasdair Allan was forced to defend National 4, which has no external exams, after a headteacher at last week's School Leaders Scotland (SLS) conference expressed fears about the prospects for pupils who did not go on to higher qualifications.
Parents' groups have also criticised National 4, suggesting that assessment procedures need to be reviewed urgently.
In a question-and-answer session at the SLS conference, David Dempster, headteacher of Edinburgh's Boroughmuir High School, voiced concerns that National 4 and 5 may have "created a kind of two-tier system". He contrasted the Nationals with the old Standard grades, which worked on the principle that "all youngsters were part of the exam system".
"Now we have created this year a two-tier system, where a bunch of youngsters seem not to be part of the exam system, and I'm not so sure it's a good way forward," Mr Dempster told Dr Allan.
Ken Cunningham, general secretary of SLS, said the lack of an external element was not a problem in itself, but agreed that many parents and employers had yet to be persuaded of National 4's merits.
Some teachers might be nervous about making judgements that decided which grade a pupil would receive, but it would be a "cop-out" if schools relied entirely on external assessment, Mr Cunningham said. Although it was still too early to judge their impact, he added, he had been "relatively impressed" with the Nationals' first running in 2013-14.
Dr Allan's response to Mr Dempster was unapologetic. "I don't accept the charge that [National 4 and 5] create a two-tier system," he said. Universities and further education colleges were entrusted with internal assessment - as were schools around Europe - and Dr Allan said he saw no reason why Scottish schools should be different.
He recalled meeting a student who had lacked confidence and was set on leaving school without qualifications, but became keen to move on to Highers after success at National 4.
"If we get this right, I think we can create pathways for all sorts of people who might never have thought about pursuing qualifications," Dr Allan added.
Parents' organisations remain to be convinced, however. "The internal assessment for National 4 needs to be reviewed as a matter of priority," said Iain Ellis, chair of the National Parent Forum of Scotland.
"Parents and pupils are unclear about why some Nationals are internally marked and others aren't. It gives the unfortunate impression that some qualifications have a higher value than others, which is not what we want to see happening."
The Scottish Parent Teacher Council has also fielded concerns about National 4, and this week published results of a survey on the new qualifications.
Many pupils had been "effectively assessed to death" at both National 4 and 5, according to executive director Eileen Prior, and parents were unhappy with the lack of information at both local and national level.
Larry Flanagan, general secretary of the EIS teaching union, said that Nationals were designed to "better meet the needs of all learners and recognise a broad range of pupil achievement" and to dovetail with the assessment processes used in FE colleges and modern apprenticeships.
"[National 4 is] robust in that all work is assessed and graded by class teachers who are familiar with pupils and their particular abilities," he added.
Earlier this year, the emergence of a "#Nat4scum" hashtag on Twitter was highlighted by James McEnaney, an English teacher who saw it as evidence that the qualification was already being dismissed by pupils.
"If you are going to maintain a system which still worships at the altar of the May to June assembly-hall exam, then any qualifications which do not incorporate that - that is, the National 4 - are, without a shadow of a doubt, going to be viewed as second-class, for `the stupid kids' or, in the worst-case scenario, essentially worthless," he told TESS this week.
Standard Grades were "awful", Mr McEnaney added, but at least they had resulted in qualifications that were "viewed on as equal a footing as possible, and that is unequivocally not the case with the new National 4 and 5 arrangements".