Middleton Primary School held an emergency meeting on Friday, where parents were told that children from two year six classes would be affected by the blunder.
The YEP understands that bundles of one exam paper, completed by pupils sitting the Key Stage 2 mathematics SATs tests in May, have been lost.
It came to light after The Standards and Testing Agency (STA), the government agency that sets, assesses and administers the exams, wrote to the school to confirm they were “unable to locate” the papers.
The national curriculum tests help measure pupils’ progress, identify if they need extra support and are also used to assess school performance.
“I’m gutted for the kids and school teachers, they’ve all done after school SATs clubs,” one parent, who did not want to be named but whose child sat the exam, said.
“The kids worked really hard, and the school gets scored on how well they do in SATs and now that’s going to be affected.”
Parents were told during the meeting on Friday that the school had receipts, confirming the test papers had been dispatched by courier to the STA.
But without the missing paper, they claim children who sat the mathematics exam at the school will incur a fail.
Another parent said: “First they tried to say that the school had misplaced the papers, but the school had proof that they sent them with receipts from the courier.
“It’s the main maths paper that’s gone missing. So because of that, all the kids in that school are going to incur a fail.”
Samantha Williams, headteacher, said: “The STA has written to inform us they are unable to locate a number of our Key Stage 2 maths test papers, which have gone missing after being collected from school.
“We’re obviously extremely disappointed and have written back to the STA to request a more complete explanation regarding the circumstances and the implications of this for our pupils.
“We will of course keep parents fully informed through this process.”
A Department for Education spokesperson said: “Following the incident with Middleton Primary School, the school has received a formal apology on the issue.
“No pupil will be adversely affected regarding their move in the autumn onto secondary school as a result of this issue.”