Gakuen no Hime Kouryaku Hajimetara Shuraba ni Natteta Ken

One day, Yuuma Kanbara finds himself in a desperate situation when his younger sister, Ayane, blackmails him with a certain cringe-worthy secret from his past. Her demand? To secure her a spot among the "Hime 6," the prestigious titles awarded only to the top six candidates in Himegasaki Academy's famous general election.

Determined to claim a throne of her own, Ayane uses Yuuma's weakness as leverage and forces him "conquer" her rival princess candidates, to clear the path to her own victory. But winning over the school's most dazzling girls, who have the entire student body captivated, is no easy task, and Yuuma struggles at every turn... Worse still, as he gets involved, he finds himself tangled in the increasingly complicated romantic entanglements of the princesses themselves!

(Source: GA Comic, translated)