King's Proposal, Vol. 1 (light novel)

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!