"I'm the idiot who put it online," said the man, named Steeven, in an interview with radio station France Info.
The discovery of the leak prompted a huge furore in June and ended up with the education minister deciding to cancel all points from the question.
The decision led to protests by students and parents that it unfairly penalized them.
The Baccalauréat, often known simply as the Bac, is taken by most French 18-year-olds at the end of their time at secondary school.
Steeven said in the interview that he had received the question by mail on June 20th. He hadn't believed it was genuine but decided to publish it anyway on a popular teenage chat forum.
"I just wanted to have a bit of fun and create some buzz on the forum," he said. The next day he realized that the question had been genuine.
He apologized to students who may have suffered as a result of his actions, although said he wouldn't be apologizing to the many people who insulted him online.
He also criticized the media who, he said, blew the issue out of all proportion and harassed him and and his family to get information.
"What surprised me is how the media made something really big out of this, when really it wasn't such a big deal," he said. "It's not like someone died."
Steeven will face charges over the leak and his case is likely to be heard at the start of 2012.