GCSE exams due to take place in early November have been postponed for almost two weeks.
Education Minister Peter Weir made the decision due to the closure of schools for an extended mid-term break.
GCSE exams run by the Council for the Curriculum, Examinations and Assessment (CCEA) will now begin on 23 November.
Almost 1,500 positive cases of coronavirus have been recorded in NI schools since they reopened to pupils at the end of August.
Thousands of Year 11 and Year 12 pupils taking GCSEs in science are due to sit exams in November.
There are exam papers in biology, chemistry and physics on three consecutive days.
Those exams had been due to take place from 11 to 13 November.
However, schools have now been told they will be postponed until 23 to 25 November.
In a statement, CCEA said that the decision had been taken by Mr Weir “following the Northern Ireland Executive’s recent decision that all schools should close for an extended mid-term break, due to the ongoing health situation in Northern Ireland”.
Pupils are also due to sit maths and English language GCSE exams in January 2021 and more science GCSE exams in February 2021.
Mr Weir had preciously decided that the main summer A-level, AS and GCSE exams in Northern Ireland will start one week later in 2021, but will still finish by 30 June.
The minister said he had asked the Northern Ireland exams board CCEA to consider what he called “back-up” arrangements.
Pupils are due to sit fewer exams in many GCSEs in 2021 but CCEA has not yet provided final details of precise changes to individual subjects.