"Fifty Shades of Grey" tells the tale of a naive college graduate's energetic trysts with a billionaire -- Christian Grey -- who has a penchant for bondage.
The Manchester Evening News
reported that Liam Scholes arrived at Sale High School in northwest England dressed as Grey, complete with cable ties and an eyemask on Thursday. The school contacted Liam's mother and told her the costume was inappropriate.
Nicola Scholes told the newspaper she and her son had thought the idea was funny.
"We were walking home from school and he had the idea of going as Christian Grey," she said. "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."
The school called her to say the costume was not appropriate and that Liam would not take part in the book day event or appear in any photographs, Scholes told the paper.
A teacher from the school had dressed as the fictional serial killer Dexter, Scholes said: "I don't see why sex is more offensive than murder."
Scholes told BBC News that her son had not read the book or watched the movie spin-off but had been aware of the media around the tale. "He understood what the cable ties were supposed to represent, as did other children of his age."
Scholes told the BBC the costume was light-hearted: "It was a laugh, it was tongue in cheek and there was no offense."
On Friday Scholes tweeted an image of Liam with the caption: "Liam's offensive book day costume, it was that inappropriate that they told him to change his character and wouldn't include him on any photos. It also required a phone call home regarding the matter.
"Yet it was appropriate for a teacher to dress up as a serial killer and other students to come in with 'guns.' I thought Christian Grey was the most talked about book character for the past few years. #nosenseofhumour."
Sale High School issued a statement
saying its actions reflected its "high standards in terms of student behavior, welfare and safeguarding."
"The student was not excluded from taking part in any of the activities as has been wrongly reported, his costume was modified and he was then able to fully participate and enjoy the day with his peers," headteacher Lynn Nicholls said.
"The aim of our world book day events was to celebrate and encourage reading and we are disappointed that what was a fantastic day for our students, has been marred by this issue," the statement concluded.