I was pretty shocked to see Jo jung suk     and that ending.

I only read source manhwa until the wolf fight so I dont know how the source ends, but its pretty clear that these creators are putting a spin on  it and only taking the skeleton of the story.

So my question , Will there be class three with   Choengang playing a major role in their lives ?

will they add suho back into the main story as a returnee to the high school since he was in coma ?

does it end with donald dying ? or does someone kill him? 

I don’t think so. And if there is ima be PISSED because there would be NO reason to rush the sources material in a single season and could’ve fleshed everything out this season and did the Union stuff in season 3

I think they can do S3 with the antagonist being the organization and Wolf working for it. On the webtoon Donald Na was killed by a truck, but here he was probably killed by the organization. They could even bring Suho back to the cast. 

They may have this in mind since they added the funeral post credits, which leaves a worse taste than the last scene being Suho reunion.

It would be completely original though, since they reached the end of webtoon, but all of the drama is original already.

 Zorro:

I interpret the last scene with Wolf and that organization's head as more of a nod to Wolf's original character traits, since he was more or less saying no to his offer because it didn't seem fun to him, and potentially to the viewers too that this is the end for now.

Yes, that totally feels like his character. But having that after Suho reunion felt so out of place, wouldn't be surprised if they have the mafia as a possible sequel plot, even if Wolf doesn't join them he could be around, like joining Eunjang to fight them.