I know School Days, and that's it. How about a couple of summaries? After all, the question was not so much any other titles that can't be made into movies, but the themes that can't be done.
Elfen Lied- Extreme ultraviolence (body parts being telekinetically ripped off ), lots of nudity, cruelty to animals, child abuse and molestation.
Gantz-Extreme Ultraviolence, sexual content.
Ghost Talker's Daydream- Sexual content (not so much the OVA but the manga since the main character's a professional dominatrix.)
Gunslinger Girl- An Italian government agency "saves" terminally ill or injured little girls on the verge of death and gives them second life as brainwashed cyborg assassins who do the government's dirty work.
Hell Girl-A group of demons runs The Hell Correspondence, a website for those who begrudge others. If the human agrees to the pact the demons send the person to hell, however, the price is users soul will also fall to hell when they die. I mentioned for the religious and moral issues it would bring up not to mention that not all the people who get sent to hell actually deserve it. Essentially, it's franchise full of hopelessness where the bad guys always win and justice is dead.
Koi Kaze- Deals with the issue of brother/sister incest.
Mnemosyne (aka Rin: Daughter of Mnemosyne)- Extreme violence and lots of sexual content, among other screwed up things.
School Days- For a sweet high school romance triangle gone horribly wrong.
When They Cry (aka Higurashi no Naku Koroni)- For extreme bloody violence committed by children. Also graphic torture of old people and children.