I like the premise, similar to prison school, but there just isn't enough going on to hold my interest. Whatever mystery there was became obvious the second they revealed all the girls are succubi. Maybe if there was a secret or slim chance of overpowering a succubus or there was something in their favor of escaping or revealing what's been happening? Something about the island being a ritual site, or their being a time or place when succubus are vulnerable?
You gotta throw them a bone. Even just the vaguest hint that a succubus can fall in love and see a man as more than food could work. At the very least go the to love-ru route and every chapter has at least 1 lucky pervert scene.