Mai-Otome (manga)

Mashiro-hime has just transferred to the all-girls school, Garderobe...but little do the other students know, she's actually a boy. Desperate to escape an all-boys school where he was looked down upon, he disguised himself as the late Mashiro-hime to get in. Unfortunately, due to "her" royal status, members of an organization known as Shwarz are trying to assassinate him, and all the while, he's trying to break his Contract with Arika Yumemiya, who unexpectedly became his Otome to protect him from a member of Shwarz.