Kateikyoushi no Lelouch-san

For some reason, Lelouch becomes C.C.'s tutor?!

Lelouch and Nunnally lived in peace, until C.C.—a spoiled young aristocrat with less common sense than a rock—moved in next door. And now at the bequest of his mother, Marianne, Lelouch has accepted responsibility for C.C.'s education and entered into a contract to be her personal tutor...?! Ready, set, go—it's a lighthearted high school AU!

(Source: Kadokawa, translated)