I have been curious to try different takes on this idea, but since it doesn't really need a GM to run, and it can be on the forums as well as elsewhere, I have decided to make this its own thread. Same rule as always: Send a PM if you're interested in doing this with me.
Anyway, the basic idea is that a religious school of some sort (High School, Private School or College) secretly harbors a sort of darkness. My character, a schoolgirl comes across this and... Well, just about anything can happen really. It can range as small as the girl learning to deal with the dark force she's come into contact with, to an adventure that goes beyond the school itself.
Here are a couple ideas for how this sort of thing can be set up. You can use them as-is or take bits and pieces to make your own thing:
Second Eden: Once Adam, Eve, and eventually all the animals left Eden, God no longer saw need to take care of it. In the intervening years, the denizens of Hell sought to make the former garden their own little haven. At the same time, God's resignation from interfering with Earth has resulted in a gradual shift of attitudes regarding Heaven and Hell: Fundamentalists and how they try to practically coerce others into worshiping the Lord, for instance. So in his twisted love for humanity, the Devil over time changed Eden into a sort of community city-college, exposing humanity to a taste of what Hell is truly like. Of course, his intent isn't revealed... not at first anyway. It would be for the best that humans and the fiends of Hell get acquainted with one another comfortably, first.
From The Abyss:Long ago, terrible demonic beings reigned the world, until a war broke out between warring lords. No winner could be determined, and eventually demonkind quietly disappeared, leaving humans to flourish in its place. A coffin said to contain what remained of a dark lord has been forgotten by time, and eventually left beneath a catholic school. That is, until a young woman discovers a crack forming in the school's basement.
The Lost School: A decrepit school once belonged to a wealthy religious leader in his prime, until he could no longer manage the school without controversial methods. Once the school was abandoned, it became a barely tiny tourist attraction while dark creatures began to reside in its place. And yet, the school still has a few secrets left to be unearthed.
The Serpent of Eden: The serpent has quietly slithered through the passage of time, living off where it stayed and even partaking in the occasional debaucherous mischief. Its most recent place of residence is at the hidden outskirts of a private academy for a change of pace.