Apocalypse no Toride - (Fortress of the Apocalypse)
Well in he absence of HOTD there is another Zombie manga which is even more freaky.
It's basic backdrop is there is this prison/school for young males convicted of crimes and the main character is sent there for supposedly being involved with some murder which he only witnessed but was sentenced for. Once there he sees all the delinquents that live there and meets his 3 roommates. Then a Zombie outbreak happens and all hell breaks loose....
APOCALYPSE NO TORIDE!