Sensei! Bokutachi ga Sekai wo Horoboshimasu.

Teachers are entrusted with the sacred duty of guiding and protecting their students—but not undercover assassin Mr. Solo. In a world where magic users and non-users coexist, he walks a solitary path as the wizard hitman "Number Zero." His latest mission: pose as a magic instructor for the unruly, bottom-tier, second-year students of Class D. To complicate things, he’s saddled with a new partner—a transfer student with the same deadly assignment. As they pursue dangerous student targets, unexpected twists arise in this exciting magic school fantasy!
(Source: Manga UP!)