A private school has vowed to “fight tooth and claw” to defend its reputation after a parent threatened to sue over his son’s failure to get straight As at A-level.

David Webster narrowly missed out on a place to study maths at University College London after his geography coursework was unexpectedly graded E, dragging his overall mark down to B.

His father, academic Roger Webster, claims pound;11,000-a-year Silcoates School in West Yorkshire breached its contractual duty by failing to guide his son properly. He blames the school because the exam board, OCR, said David’s coursework did not meet its published criteria.

All six pupils in David’s geography class got lower than expected marks for their “personal investigative study” unit, and despite two appeals all scripts came back unchanged.

Darryl Wideman, headmaster of Silcoates, said the teaching had been “excellent”, as shown by high scores in other modules.

“We have always maintained the exam board moved the goal posts. I’m told it’s a practice in some exam boards to downgrade coursework so there aren“t too many As. If he (Dr Webster) has a dispute perhaps he should pursue the exam board,” said Mr Wideman.

David Webster, 20, is now reading natural sciences at Durham University. His father, a tutor at the Royal Northern College of Music in Manchester, has spent thousands of pounds on legal fees, attempting to force Silcoates to accept it was at fault.