Master School Olympus

A young, spunky, fearless girl named Sang is searching for the Demon School Hades. She runs into a group of misfit monsters who are all ditching classes from the same school. Sang convinces them to take her to the school ... and, since only monsters are allowed in, she 'monsterizes' herself by letting one of the misfits - a vampire - bite her. Sound pretty cool? Then welcome to the world of PhD!
(Source: Tokyopop)
There are currently 10 volumes published, and the series' original publisher, Daiwon C.I. lists it as ongoing, but no new volumes have been published since 2005.