Royal Mail has apologised to anxious pupils whose grammar school entrance exam results were delivered late.
Almost 14,000 schoolchildren were waiting to find out if they scored enough to secure a place in a school of their choice.
However Royal Mail admitted a small number of results did not arrive with normal deliveries due to a sorting error.
"The vast majority of results have been successfully delivered this morning," said a spokeswoman.
"A short delay to a fraction of items, mainly in the Craigavon area, will have now been delivered.
"We know this is an anxious time for parents and children and apologise for any concern caused."
The unofficial controversial transfer tests replaced the 11-plus.
About 7,000 children sat one exam for non-denominational grammar schools, while another 7,000 took the GL assessment, mostly used by Catholic schools.
Some students sat both.
It is understood those affected by the postal problem took the GL assessment.