Angel Beats!

Upon his death, Yuzuru Otonashi awakens into an afterlife high school where a group of student rebels, lead by Yuri Nakamura, take arms against a dispassionate adversary with supernatural powers whom they call Angel. Calling themselves Shinda Sekai Sensen (a.k.a. Afterlife Battlefront), they're in open rebellion against God for their crappy, unfulfilled and cruelly unfair lives (and against a projected reincarnated future as meager sea life, presumably barnacles or water fleas). Though no one can die since everyone's already dead, killings are nonetheless very painful experiences, with complete recovery to follow. The greatest fear in their struggle to maintain their humanity is obliteration (Angel's goal for them), from which there is no return.