An independent report into the introduction of new National qualifications this summer found there had been a "significant and unsustainable" level of over‑assessment in schools.
The review, commissioned by the Scottish Government, concluded: "This increase in assessment was not intended and requires to be addressed at both national and local level."
The report was welcomed by Michael Russell, the education secretary, who has written to schools highlighting the need for change. His letter states: "It is vital we learn from our experiences. The national education bodies are already taking steps to implement these recommendations and will continue to support teachers and schools throughout the year ahead."
However, Larry Flanagan, general secretary of the Educational Institute of Scotland (EIS) teaching union, described the report as "shallow" because it did not blame the Scottish Qualifications Authority (SQA) for the problems.
He said: "The glaring weakness in the report is the shallow analysis of the experience of the past session. EIS members were very clear the SQA had failed to deliver the level of support which schools needed.
"Late changes to assessments, poor communication and lack of professional support have contributed greatly to the workload pressures experienced in schools."
The report, by Kenneth Muir, chief executive of the General Teaching Council of Scotland, called on the SQA to review the amount of assessment in schools and the administrative burden on teachers. Councils and senior education officials have been urged to ensure teachers get better classroom support.
The report says schools should be more flexible about when pupils take exams, in line with the aims of the new Curriculum for Excellence. Schools and teachers have been asked to collect "appropriate and proportionate" assessment evidence to minimise pressures on pupils.
Mr Muir said: "Introducing new qualifications is always a complex, time consuming and challenging process. What is important is that we continue to learn from issues that arise and build on the good practice seen in introducing... the new qualifications."
Iain Ellis, chairman of the National Parent Forum for Scotland, said: "We need to learn from the experience of last year and make changes where necessary."
Earlier this month, the National exams, which replaced Standard Grades, were hailed a success with pass rates on a par with similar exams in recent years. However, the exams followed a year of turmoil in schools with surveys highlighting high levels of teacher stress.