The book sat unobtrusively on school library shelves in Wanganui until last week, when librarians had to remove Ted Dawe's Into the River after an interim ban was imposed.
The young adult novel had an R14 classification until August 14, when the restriction was removed.
The interim ban follows protest from lobby group Family First.
Wanganui High School librarian Sue Goldsbury said the young adult novel had been in the school library collection since 2013 and had not caused any fuss.
"Although it had the R14 classification when it arrived, we decided the content was not suitable for younger readers and classified it as suitable for Year 13 readers only."