This Is the Wizard's Secret Weapon, Vol. 1 (light novel) — Taro Hitsuji

Rix, a master swordsman, is tired of bloody battlefields—so he fakes his own death to go into retirement! He starts fresh by entering a magic school...despite having no magical aptitude whatsoever. However, he's ready to work hard alongside the scholarship student Shino, undeterred by her cold attitude toward him. Thanks to his skill with a blade, he overpowers his magic-wielding peers, inadvertently smashing through anything that's common practice at magic school until he catches the eye of the Imperial princess. She wants to put him back on the battlefield, but he doesn't want to be the strongest—all he wants is to live a peaceful and enjoyable life!