There was a time when there were ninja serving the powerful, hired for intelligence missions, destruction, or assassination. While the times have changed, the need for ninja has not vanished. In a particular school, there are girls who are secretly in training to become ninja. Ninjas-in-training, Asuka, Katsuragi, Hibari, Ikaruga, and Yagyuu each have a secret technique scroll that they protect at all times. This features their tumultuous school life as they pursue the path of the ninja!

[Source: AniDB]

Special, unaired, episodes released with the Senran Kagura BD/DVDs.

A letter arrives at Death Cram School. Inside is a an invitation to a spa resort.
Yumi declares to the four excited girls that, "No. This is evil itself," points out that they are slacking off too much. Shiki proposes, "When opinions are divided, it's time for boob rock-paper-scissors!" and then wins so they all go to the pool.

There, girls from Hanzoo Academy, Homura Crimson Squad, and Hebijo Clandestine Girls' Academy also come, but Asuka says, "Let's have fun together today" and they call a temporary truce.

However, a trivial matter sets off a sour quarrel and, at Yumi's suggestion, they settle it with a showdown in the style of the ancient shinobi.

[Source: Anime News Network]

Girls study at schools for modern shinobi. They dedicate their entire young lives to learning to become true shinobi, battling each other to polish their skills. And then one day, Asuka, whose grandfather is a legendary shinobi, encounters an ancient evil being named a "Youma" in the back alleys of Tokyo at Christmastime. Where there is light, there is shadow. Over 28 of the strongest and most well-endowed ninjas in history, both good and evil, will collide in a battle to determine true justice. And this deadly battle begins... right now.

[Source: Crunchyroll]