Nicola Scholes dressed her son Liam as the billionaire character in the blockbuster novel because she thought it would be 'funny'.
But teachers at Sale High School in Great Manchester didn't see it that way and called the 36-year-old mum to say he'd been banned from school photos of the event because his costume was ruled inappropriate.
Nichola told the Manchester Evening News: "We thought it was funny. We were walking home from school and he had the idea of going as Christian Grey.
"At first we laughed it off but then we discussed it with a few friends and saw the funny side and decided it was quite a good costume idea.
"He went in a suit and shoes and with an eye mask and cable ties.
"The cable ties were tiny and not enough to do anything with them. We didn't think there would be a problem.
"I got a phone call home from the school saying the outfit was inappropriate and that he had been excluded from taking part or being in any of the photographs.
"Me and the teacher agreed to disagree and it's not being taken any further but I was disappointed for Liam that he'd been stopped from taking part and that he's had a dressing down.
"One of the teachers went dressed as Dexter and I don't see why sex is seen as more offensive than murder.
The school held the World Book Day celebrations by asking Year 7 children to come dressed as famous literary characters.
The Fifty Shades of Grey series – which is notorious for its explicit sex scenes - has sold more than 100 million copies worldwide and tells the story of a relationship between college graduate Anastasia Steele, and a young business magnate, Christian Grey.