Fourteen-year-olds at Katharine Lady Berkeley school in Wooton-under-Edge, Gloucestershire, have had their English tests re-marked in each of the past three years.
And each time more than 50 pupils' results rose by at least a level.
Last year some pupils who had failed originally to reach a level were then awarded a level 4.
Robert Penman, head of English, said teachers, pupils and parents had all suffered.
"What does it do for pupils who, after a year's work, get a grade N? It is humiliating for moderately bright pupils. What basis is that for them to go on to do GCSEs? We have had to launch a massive campaign of reassurance," he said.
Three teachers in the English department spent a week between lessons and after-school preparing its appeal. Mr Penman said he was lucky that markers on his team were able to spot errors quickly.
The school also had to convince inspectors who visited in February that the performance data they were using was based on incorrect results.
Mr Penman said: "It is getting much easier to convince them there has been a mistake.
"If we ever get a year when results are right, then parents probably won't believe it."