Maken Ki! is an anime about an all girl’s magic school that decides to start accepting male students. One of these students is Takeru.
In the second episode of the OAV they have gone on a training camp at a tropical island. One evening the teachers invite the boys round for sake and get a bit drunk. One of the teachers begins to seduce Takeru, but one of his female schoolmates walks in on them and she thinks he’s just being a pervert so zaps him with some unknown magic blast.
When he wakes up he finds he has become a girl! He uses this opportunity to join the other girls in the bath and infiltrate their sexy slumber party… until later in the night his real identity is discovered.
A fun gender swap anime episode with plenty of ecchi loveliness. 
good