Kamen no Kurokishi. Shoutai Bareta node Mou Gakuen demo Musou suru

The mysterious, strongest knight known as the "Black Knight" saves people in times of crisis. He was supposed to live as a masked hero in the shadows—yet his true identity is exposed as none other than Adele, the disgrace of a marquis family! Though Adel pretended to be incompetent in order to live a lazy life, his cover is blown, and now even at the academy he can no longer conceal his true power...!? A school tale of unrivaled dominance featuring a knight whose secret identity has been revealed!

(Source: Kadokawa, translated)