Pupils are at risk of receiving unfair and incorrect marks for their Sats exams this summer because of problems caused by the introduction of an online marking system, whistle-blowers have told TES.
Markers submitted marks that they later realised were "wrong", but they were unable to retrieve and correct them under the system that is being used to electronically mark all Sats papers for the first time this year.
Pearson, which administers the national tests for 11-year-olds, said it was confident that its "exacting standards" were being met. The company added that it had a "number of ways to review questions if we think there is an issue with marking".
But markers told TES they did not think the checks were comprehensive enough to pick up all the problems that the online marking system was letting through.
Tony Draper, president of the NAHT headteachers' union and head of Water Hall Primary School in Milton Keynes, said his members had also been told that the system was causing papers to be marked inaccurately. In light of the concerns, the union is urging schools to check through pupils' papers more closely than ever before after the results are published in July.
"Accountability is high-stakes and we don't want schools getting into difficulty, or forced to become academies, because of marks that may have been affected by possible glitches in the system," Mr Draper said.
One whistle-blower, who has been marking English papers, said that markers received an email four days after they had started, giving them new information about what marks they should award for certain responses.
This made it clear that some of the marks she had already submitted were incorrect. But the system would not allow her to retrieve the marks. The whistle-blower believes this is likely to lead to some children and schools receiving incorrect Sats results.
"I can think of [questions] I've marked as `no score', but the awarding body has then said, give them a mark," she said. "But I'd already submitted those marks. I can't get back to them.If you had the [paper] scripts there you could go back and make the adjustment.
"I think the system means a lot of marks have gone through that are wrong," she said. "It's quite possible that schools will [see] a difference across the class, depending on whether papers were marked before or after the clarification.
"If, when they get their marks back, one child in the class has got a mark for something and another child hasn't, the school can question it."
Pearson has admitted to TES that tens of thousands of questions that were subject to clarification had already been marked by the time the email was sent.
But a spokesman said the original mark scheme "remains correct as when it was issued", adding: "Where helpful we reinforce points from the training, supervision and guidance to markers."
However, the whistle-blower said the mark scheme did not provide the same level of detail as the clarification.
She told her team leader - a more senior marker - that she was concerned some children had been given the wrong marks. The team leader agreed that there was the potential for inaccuracies. But they were unable to re-mark everything that had been completed before the clarification was issued because the online system allowed them to access only the 20 most recently marked questions.
Computer says no
Another whistle-blower, who has been marking maths papers, said that technical problems with the system had resulted in the wrong marks being entered.
"I know there were questions marked wrong," she said. "There were cases where I put a mark in for a question and was waiting for the next question [to load], and there would be some sort of glitch where it took the mark I'd given for the previous question and submitted it.Then there was an error message." She could not get back into these questions to change the marks.
Both markers were uncomfortable about having put incorrect marks into the system and said they were not confident the errors would be picked up and corrected by Pearson.
Pearson said it had several ways of addressing potential marking problems, including "real-time reviewing of marking by senior markers", the use of "seeded" questions, random sampling and "responding to marker feedback requests and targeting items for review or re-mark between specific date ranges, or from specific markers or teams".
"It is simply not true to suggest that the marking isn't accurate," the spokesman added. "It is also untrue to say that we cannot review and ultimately change marks where necessary, if a marker raises an issue with us."
`One mark makes a massive difference'
Even if only a small number of marks are incorrect in this year's Sats, it will cause problems, according to Amanda Hulme, headteacher of Claypool Primary School in Bolton, Greater Manchester.
"One mark can put a child above or below a threshold, and that makes a massive difference both for the school and the child," says Ms Hulme. "There's so much restriction on schools in the delivery of the tests.My job is on the line if there's a mistake.
"It seems ludicrous that there's that kind of scrutiny, but that it seems OK for the odd mark to be wrong."