Itoshiki Nozomu was actually DEAD and he was sending ghosts to heaven as a form of graduation. So, all the student we have seen was already DEAD. Zetsubou shita!
But actually, they were all alive (???). Instead, students all had experience of attempting suicide. They were all taught by Nozomu for their new life. Medetashi medetashi
...and suddenly, Kafuka is supposed to be DEAD; Abiru has Fuura's eye, Chiri has her heart, Kiri has her lung. Kafuka we know was actually other characters who were acting like her. Zetsubou shita!
They do not remember who gave them their organ, but Fuura Kafuka continues to live on inside the girls
And actually, Fuura was alive. This does not make any sense. Zetsubou shita!
At the end, Despair sensei marries to Fuura.
Nice ending, actually.