SZS Anime Info ₊˚⊹ ᰔ
As of right now, there's only 3 seasons and 3 OAD's for SZS. Sayonara, Zetsubou Sensei was adapted into an anime on July 8th, 2007, courtesy of SHAFT animation. The last special was released on January 31st, 2012. Only around 100 chapters were adapted, so the ending of SZS isn't adapted yet. As of right now again, SHAFT has not made any statements about continuing SZS.
Regarding of the watch order of SZS, it can be very confusing to newcomers due to the different names of each season. I'm going to quickly clear it up, just because I was confused when I first got into SZS :3. The first season is just referred to as SZS, it doesn't have a special name. The second season is referred to as Zoku. The third season is Zan. For the specials, they're just extra episodes basically. The first special is called Goku, it comes after Zoku and before Zan. The next one is Banigachi. This comes after Zan and before the next special. The last special doesn't have any special name and is only a blu-ray exclusive. This was all just word vomit, so to make it easier to understand I'll just put it into a list :).1. SZS (first season)
2. Zoku (2nd season)
3. Goku (OAD 1)
4. Zan (3rd season)
5. Banigachi (OAD 2)
6. Special (OAD 3)There are also specials called Jo, which you don't need to pay attention to due to the fact they're just season recaps. There are also two other specials called Omake and Shuukan Zetsubou Sensei Hihyou, which are nearly impossible to find but are also unnecessary, so you can skip those two. It's kinda hard to watch SZS legally since there is no apps that have it, so to watch it you either have to go to illegal websites or buy the blu-ray. Only the first episode is available to watch on YouTube. There are also multiple different openings for each season strangely, going up to around 4 for one season. There are also rap versions for the three main openings.
SZS Poster
Zoku Poster
Zan Poster