Mr Tuck, chief inspector of schools and mastermind of the curriculum and assessment reforms, said schools and colleges should have the bones of the programme in place for the new start date which the Government has postponed a year. However, new levels and courses would have to be phased in according to local circumstances, he said.
Helen Liddell, Labour's education spokeswoman, is committed to reviewing the reforms if she becomes minister but a string of senior Scottish Office officials maintained there would be no further back-sliding.
Mr Tuck deflected suggestions of a second postponement and fended off criticism from the teacher unions and local authorities about timescale and resources.
He said: "From August 1998, all Higher Still qualifications, with the exception of the Advanced Higher, will be in place. As a national menu, there will be no alternative to Higher Still qualifications."
He anticipated that all current Higher courses would be unitised and all modules turned into the new Higher Still format. Some phasing would be essential "because it is up to schools and colleges to decide the pace at which they take advantage of the menu of qualifications," he added.
He said: "My guess is that in most schools we're talking about a five-year period before Higher Still arrangements will mature. But with minimal implementation in the first year, there will be benefits."
Mr Tuck said there would some leeway for students embarking on two-year Highers. The general Scottish vocational qualifications would also be phased out.
Gerald Wilson, head of the Scottish Office education and industry department, reiterated the Government's determination to press ahead. "Pupils should not be denied the benefits of Higher Still for a further year. It needs to be implemented on time in 1998. Higher Still is in good shape."
Mr Wilson admitted the reforms would "accommodate a range of responses", describing Higher Still as a menu schools and colleges could choose from.
Mr Wilson said that more than Pounds 10 million will have been spent on Higher Still by the end of next year when the development phase will be well under way. That did not include additional funding for the Scottish vocational education council's GSVQ programme.
He said Higher Still had been "the most comprehensive exercise in consultation in Scottish education" and that "it had shown itself able to respond to concerns of practitioners and able to make adjustments where necessary". The Government had listened to concerns about assessment, availability of teaching materials, guidance and staff development.
He was backed by Douglas Osler, senior chief inspector, who added: "There's a degree of consensus about the aims and principles of Higher Still and that's something we should hold on to when we're discussing the rocks on the road. "
On a whole range of details, the Government had accepted changes to the reforms. They included delaying the start for a year, putting back the final external exams to create more space, re-examining core skills, amending aspects of individual subjects, re-assessing provision for able students, responding to further education's concern about external assessment flexibility and addressing workload worries by creating a national bank of assessment items.