Miss Savage Fang, Vol. 2 (manga) — Kakkaku Akashi

Envil, the Savage Fang was the strongest merc around. They called him a magicless pleb, but it never stopped him from opening their throats with his sword -- until he died and reincarnated into the body of Mylene, the same tyrant he helped depose! He might have her body and her magic, but he'll need to learn how to use it first. Flanked by the spineless boy Mylene's parents had arranged for her to marry one day, and princess Colette from the neighboring empire, they enroll at the Magic Academy! Sure, the homework might be boring, but missing persons and a viral new drug mean there's a lot more to this school than meets the eye...