Nisekoi: OVA

Magical princess and ruler of the Land of Magic, Yui Kanakura, tasks Kosaki and her comrades to exterminate Doctor Maiko for good. During their last stand, Yui provides the girls with more protective yet skimpier outfits to shield themselves against Doctor Maiko's attacks. However as Doctor Maiko proves to be impervious as well, sage Rurin steps in and easily defeats Doctor Maiko. With peace restored and back to their everyday lives, Rurin surprisingly appears before Kosaki assigning her with a new task and 'transforms' her in the middle of her class, appearing naked before her classmates. (Source: Wikipedia)