Gedou Gakuen

both anime have a dark horror theme that takes place at school 

It's almost a reinterpratation of Urotsuki. Aslo a hentai with a plot. Good balance between sexy stuff and storyline. From the same creator. 

They're both centered around a dark coming of age story with a demonic possession as a metaphor for adulthood, with both the erotic and violent content being strongly present. Rivalry in forms of other beings such as angels and demons show up with en emphasis of controlling one's urges as well of the destructive powers one has gained. Romance and loss are present as well since Nightmare Campus was heavily influenced by Devilman.