Rosario+Vampire [Manga]

Failure Human Tsukune is unexpectedly put into a school for monsters, where he meets the cute vampire Moka. He later meets succubus, Kurumu, a young witch genius, Yukari, an ice girl, Mizore, and Ruby a second older Witch Adept, Ruby. All of them fall in love with him, causing numerous problems throughout the school. Tsukune has to survive many jealous male monsters along with a gang of rebel monsters trying to destroy the school. All the while trying to survive his unexpected harem fighting amongst themselves for his undivided attention.

Similar in these categories.

Elfen Lied

Rosario+Vampire

Sekirei

Asu no Yoichi!

Ouran High School Host Club

Clannad

Vampire Knight

Toradora!

The Melancholy of Haruhi Suzumiya

Soul Eater

Girl's High

Full Metal Panic? Fumoffu