Senran Kagura OVA

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)