Jibun wo SSS-kyuu dato Omoikondeiru C-kyuu Majutsu Gakusei

The strongest man in school usually hides his true strength and acts like an ordinary student... There was a man who staked his life on such a delusion: Shion Ichinose, an ordinary student attending Crawford Magic Academy. Shion continued to behave like an "SSS-Rank" both inside and outside the school, but his eccentric behavior eventually involved those around him...? Only his delusions and acting are SSS-Rank!

(Source: Overlap, translated)