Kuchidome no Gohoubi wa Dansou Otome to Ichaero desu!

Kay Brennan is a hero, no doubt about that. They're the strongest person in their school in terms of combat strength and magical power. Compared to Kay, Dino Turner - the eternal 2nd place - might as well be nothing, and as such Dino has declared Kay to be his archrival. However, Kay has a secret: she's actually a girl crossdressing as a boy. And Dino is the only one who knows her true nature.

(Source: MangaDex)