Around 30 pupils have received the wrong results for their transfer tests.
It's understood the pupils affected had taken the test at the Rainey Endowed School in Magherafelt.
The pupils had been awaiting the test marks for the GL Assessment monitored exam, which was taken last November.
Some pupils received incorrect marks, while other letters also included the wrong date-of-birth and gender.
Rainey Endowed principal Mark McCullough told the BBC that the vast majority of pupils affected had their marks increased.
"Some stayed as normal but unfortunately there were six or seven pupils who were impacted, and we're working closely with them and their parents to ensure that it's sorted," he said.