I will not spoil the ending of the Sayonara Zetsubou Sensei manga. If you are interested in spoilers, you can check out THIS thread or the following hilarious threads from when the final chapter was released: 1, 2, 3. Even better, try finding the final chapters online (I suggest starting from chapter 280 (not every chapter is translated anyway)).
I just want to say that Kouji Kumeta is a bloody genius.
We all know Sayonara Zetsubou Sensei as that dark comedy with a negative teacher and his whacky students. It made fun of a little bit of everything in our own society and eventually got three anime seasons with a few OVAs by Shaft, a perfect fit for adapting the series.
But little did we know that Kouji Kumeta had something far grander in mind. Ever since chapter 1 his plan had been set in motion and it was not until chapter 300 his grand scheme was revealed. The answer had been in front of us all the time, with multiple hints to help, but we never saw it.
We had been fooled.
For seven years.
It was an ending that turned the manga upside down and completely changed the perspective of the series as a whole. Never will I be able to look at it with the same eyes again.
This ending turned Sayonara Zetsubou Sensei into something more.
Sayonara Zetsubou Sensei left every reader in despair.
Even crazier is that Shaft, unless they possessed supernatural powers, knew the ending all along.
Let me tell you this.
Every opening of the Sayonara Zetsubou Sensei anime spoils the ending of the manga.
But we never noticed.