Ohime-sama wa Tokkun Chuu!! ~Seinaru Mahou Shugyou~

Kingdom second princess Celestina is chosen as competitor at a magic tournament where only girls can participate. She is able in magic art, but she can not refill and control the source of magic by will. Main character is hired as her instructor to teach her various sexual pleasure techniques and thus self-control.

[From vndbreview]