Kuusen Madoushi Kouhosei no Kyoukan

Kuusen Madoushi Kouhosei no Kyoukan summary:

The story is set in a world where humanity, driven off the land by the threat of magical armored insects, now live in aerial floating cities. Thus wizards—aerial combat mages who fight the insects with magical powers—came into being.
Kanata Age is a young man who lives on the floating wizard academy city of “Misutogan.” He was once celebrated as the “Black Master Swordsman,” the elite ace of the S128 special team; however, he is now despised as the “traitor of the special team.” One day, he is assigned as the instructor of E601, a team that has suffered ten consecutive defeats. E601 has three girls—Misora Whitale, Lecty Eisenach, and Rico Flamel—with one or two peculiar quirks.