Based on a shounen manga by Ikeda Akihisa, serialised in Monthly Shounen Jump.

By a bizzare coincidence, Aono Tsukune has accidentally gotten himself admitted to a youkai acadamy, 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.net]