Headteachers have been questioning their school's Sats results - after the headline figures appeared to exclude some children from the calculation.
Initial reactions from headteachers, who were able to access their own results from 7.30am today, were that the summary figures did not seem to match the individual pupil data.
It appears that children who were working below the level of the test - who are given a label of B - were not included in some calculations.
The DfE has been contacted for comment.
The frustration came after delays getting onto the site as thousands of headteachers poised over their keyboards at once.
But once the results were known, at least some schools had good news.
Around 600,000 10- and 11-year-old pupils took the tests in reading, maths and spelling, punctuation and grammar (Spag) in May this year.
At the time the reading paper was seen to be fair, the spelling part of the Spag test was deemed difficult and the last of the three maths papers a “nightmare”.
Children are also given writing scores, but these are the result of teacher assessments, rather than a test.
Although the official release time to schools was 7.30am today, an error meant that some schools were able to see their own results for a short time on Friday. It is not clear how long schools were able to access their results before the blunder was rectified.
In previous years, schools have been able to access their results at midnight and some people were missing the old regime.
Today's scores come after one teaching union revealed that children are having nightmares about the Sats tests.
A survey of 1,200 primary teachers, carried out in June and July, by the NEU teaching union found that nine out of 10 primary school teachers felt Sats were detrimental to children’s wellbeing.
The Department for Education said in response to the survey that it trusted schools not to put undue pressure on children when administering the assessments but added that assessing the extent to which children have grasped what they have been taught is an important part of education.
The national results will be published at 9.30am today.
