Angel Wars

From ShoujoMagic:

"Forgive me. I can't become a servant of the Lord yet," Maria says as she runs from her vows ceremony at the convent where she was orphaned 16 years ago. She has received another letter from the mysterious "G," her faceless benefactor. She is not as alone in the world as she thinks; "G" writes that she has a relative at Hakuhou Academy who is in need of her guidance, but he cannot tell her this relative's name. Trying to enroll and search the school presents Maria with a number of problems, however -- the foremost of which is: Hakuhou is an all boys school!