Young people across Scotland will sit a "confusing" patchwork of old and new Highers next year because of the flexible introduction of the new exams, TESS can reveal.
A combination of old and new Highers will be offered in almost all local authorities, a situation that parents' groups called "extremely difficult", especially for students.
The Scottish Secondary Teachers' Association (SSTA) said that a picture of "complete confusion" surrounded the exams.
Unions have also reported that some teachers have lodged grievance procedures with their local authorities after being forced to implement the new exams before they were ready.
A survey by TESS reveals that the majority of councils will be adopting a mixed approach. Students in some subjects at some schools will be sitting the old Highers, while in other schools and subjects the new exams will be taken.
But in certain areas whole schools will delay implementation of the new exams until 2015-16. As a result, students in neighbouring councils, or even neighbouring schools, may sit different exams in the same subject next year.
East Renfrewshire was the only one of the 30 local authorities surveyed to say that it was holding off from introducing the new Highers in all its schools for a further year.
Computing science and accounting are among those subjects where the new Higher will not be brought in until 2015-16 by some local authorities.
Originally, it was planned that after the introduction of the new National 4 and 5 qualifications last year the corresponding Higher exams would be introduced universally in 2014-15.
But when serious concerns were voiced by unions and local authorities, education secretary Michael Russell said there should be some flexibility to allow schools to prepare.
However, unions and parent organisations have raised concerns over the huge level of variation created by the government's flexible approach.
Alan McKenzie, acting general secretary of the SSTA, told TESS that the situation next year would be "confusing, most importantly of all for the young people".
"It paints a picture of complete confusion," he said. "You could understand it if, for example, all history departments were saying we are not ready, but that is not the case. Some are saying we are ready, and some are saying we can't do this."
And Eileen Prior, executive director of the Scottish Parent Teacher Council, said the results of the TESS survey were not unexpected, but that it was "not a situation we are content with, and [it's] one which we feel will be extremely difficult for young people and their parents".
"Schools will have to work hard to make sure there is good quality communication to explain the route being taken and why," she added.
But a spokesman for the EIS teaching union said that once the option of a one-year delay across the board had been ruled out by the government, "a school-level opt-out was the best alternative option".
"Any major programme of change will always bring challenges," he argued. "But the option to allow individual departments to make a determination based on their own level of preparedness and the needs of their pupils has helped to alleviate the problem of schools potentially moving to the new Higher before they are fully prepared."
John Stodter, general secretary of the Association of Directors of Education in Scotland, said the overriding consideration was for students to be fully prepared by "confident teachers".
He added: "These decisions are taken locally in consultation with the education authority and in the best interests of young people: the old and new Highers have equivalent value.
"It would have been unrealistic and unsafe to insist that all teachers and all pupils adopted the new Higher given the scale and complexity of whole-system change and with the variety of local circumstances across Scotland."
A government spokesman said the most important thing was for each school and authority to work with parents and pupils "to make use of the flexibility in the way which best serves the interests of the pupils".
"It is vital, too, that schools keep parents and pupils fully informed of the decisions taken," he said.