About 150 pupils due to complete GCSE English and science exams have had them cancelled because their school has problems with its water system.
Students at Brynmawr Comprehensive School sitting their final tests of the WJEC examination board papers will now be graded on earlier coursework.
A-level students due to sit the exams on Thursday are unaffected.
Pupils at the school near Ebbw Vale who are in their first year of GCSEs will have the exams rescheduled.
The announcement was made by the chief executive of the exam board Gareth Pierce.
He said affected students would now have their final result based on previous coursework and exam results.
The school is to be consulted on the marks given to the pupils.
Some of the students have more work in their subjects to complete - and the marking of this will be taken into account when the results are finalised.
Pupils in their first year of their GCSEs will have any exams due to be taken tomorrow rescheduled.
The school is closed on Thursday because of problems with its water system.
A- level exams will continue as normal off site. The school should open as usual on Friday.
One 16-year-old GCSE pupil said two of her exams had been cancelled: "I feel that all my revision has gone to waste."
She added: "Surely this is not fair? I have contacted the education autorities, but there is no alternative."
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