One of the previous Nobuaki's friends is showed naked in the first episode, despise the nipples aren't visible.

Natsuko forced a classmate to have sex with her (offscreen, but showing a condom with sperm).

In one episode, Nobuaki forces Naoya and Chiemi to sleep with her, which implies he is forcing them to have sex, but Naoya refuses to do this as he thinks this as betraying them both.

Most of the students dies violently in the anime (bleeding, decaptation, dismemberment).