Nintama Rantaro the Movie: Ninjutsu Academy - All Hands on Deck!.

The Ninjutsu Academy students return from summer break to hear that due to one of Komatsuda's mistakes, all their summer homework was mixed up. As a result Kisanta, who received an assignment meant for a sixth year, has gone missing after trying to sneak into Oomagatoki Castle to steal their lord's fundoshi. A group including Shinbei and Kirimaru is sent to rescue him, but since Oomagatoki is at war with Tasogaredoki Castle it might be a difficult mission.