Student David Webster was unable to go to University College London as a result.
His father Roger said the us$18 106 per year Silcoates School in Wakefield failed to guide him properly over his geography coursework.
David, from Darton, Barnsley, obtained As in maths and further maths but only got a B in geography after he was marked down on his coursework.
Webster (50), a lecturer at the Royal Northern College of Music in Manchester, says the school should not have let his son submit the coursework.
David (20) is now studying for a natural sciences degree at Durham University but his father is still pursuing the case and has already spent thousands on legal fees.
Webster said: “David was predicted As in all his subjects and has always done really well at school.
“He went to get his results and we had a phone call to say he had got a B in geography. It meant he couldn’t go to UCL as he needed three As.
“What had let him down was the geography coursework that had been submitted. It came back marked E which was a shock to him and me as well. It meant his grade dipped to a B.
“I told him not to panic and I would speak to the head. The head assured me it was wrong and he would ask for it to be remarked.
“All six on the geography course had lower than expected marks and the school said they would send them back to the exam board. They all came back unchanged.
“When I queried this with the head he apologised and said he couldn’t discuss individual cases. He still maintained the work was of a sufficiently high standard but said the appropriate marking criteria had not been met.
“I then spoke to the exam board who said the coursework did not meet the criteria set by the exam board. They didn’t question the quality of the work, just the structural content.”
Webster went through the Silcoates appeals procedure but eventually got a letter saying the matter was closed.