More than 600 children in year six at primary schools must resit part of their selection examination for Skipton's two grammar schools on Thursday.
The error arose after a headteacher made a mistake over the times allowed for part of the test.
The mistake was put down to a genuine human error over the procedure for the non-verbal reasoning. Normally 50 minutes is allowed in total for the test, which is in four parts with varying amounts of time for each set of questions.
The headteacher realised the error after conducting the test and informed the education authorities.
The tests cream off the top 23 per cent of local schoolchildren who are awarded a place at Ermysted's Grammar or Skipton Girls' High with borderline cases resitting the examination. Further places are allocated according to an appeals procedure - around 28 per cent of pupils from the Skipton vicinity are awarded a place. News of the resit was sent out to parents last Friday.
Glenn Allgood, of North Yorkshire education department, said any distress or inconvenience was deeply regretted.
"We have looked at this at some length to see if there is a need for all of the children to be tested again," said Mr Allgood. "We took advice from the National Foundation for Educational Research - the body which produces and marks the tests - but there was no way this can be resolved and be fair to every child without retesting all of them."
Those children deemed to be in the border zone were due to retake the test today and the authority has admitted that the timetable will be put back, with border zone tests now taking place early in the New Year.
"We aim to advise all parents by January 15 if their child is deemed suitable or not for the grammar school," said Mr Allgood.
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