Ousama no Proposal

Saika Kuozaki is an all-powerful witch with looks to die for. She's also the only woman who can thwart the mysterious entities attempting to destroy the world once every 300 hours. When Saika is mortally wounded, however, she bequeaths both her powers and body to the normal high school boy who happens upon her, Mushiki Kuga. But just because Mushiki has her abilities doesn't mean he can control them. To stand a chance of saving the world, he'll have to attend an academy for mages as Saika—all while keeping his identity under wraps!

(Source: Yen Press)