The Familiar of Zero S00 E01 "Rondo" of Princesses OVA

Saito and Osmond come across a chest from Earth filled with bikinis. After examining the treasure trove, Osmond decrees that the students will take a trip to the beach! While there Osmond instructs the girls to change into various kinds of bathing suits and partake in other “rituals” pertaining to a beach outing.