Nana to Kaoru: Koukousei no SM gokko

The story takes place in the third year of high school, after the ending of the manga, but before Black Label

Note: This series is also sold in compilation volumes under the title Nana to Kaoru Last Year, but the original serialization in the Harem magazine still retains the title Nana to Kaoru: Koukousei no SM gokko, so that will remain the main title.