Rosario to Vampire

Summary

By a bizzare coincidence, Tsukune Aono has accidentally gotten himself admitted to a youkai academy, a school attended by monsters. Just when he thinks he'll run away from there, he meets the beautiful girl Moka and his mood turns ultra happy. However, she transforms into her true form when the rosary on her chest is taken off, she's a super vampire!

(Source: AniDB)