User blog comment:FairyRaven/Fairy Tail Chapter 331: Natsu's Strategy, Review and Commentary/@comment-112.203.69.115-20130517140658/@comment-7418318-20130518082210

If I remember correctly Mashima wrote about it in one of the commentary pages that you find at the end of volumes.

In school he once drew a hentai manga at the request of a friend. But a teacher busted him, chewed him out and showed it to his mother.... resulting in even more chewing out.