For the first time reader, I think that there is a level of surprise in the brutality and savage nature of the ending. The fact that the social setting would willingly proceed with the death by stoning of someone is fairly surprising. Add to this the banal manner in which it is seen and how children start to gather rocks to begin the process of community- sanctioned murder and surprise is a good way to describe the ending of the story.
What might be the most surprising element of the ending is the role of Tessie, herself. This is something that might take multiple readings to gauge. Tessie is eager and willing to be a part of this savage activity at the start of the narrative. She apologizes for being late. She engages in superficial conversation with Mrs. Delacroix. She is very happy to be a part of the community. The suprise is evident with how quick the transformation from "insider" to "outsider" can actually be. In a few moments, Tessie is changed from one who "could not be late" to a figure left pleading, "It isn't fair" and "It isn't right." This change is sudden and intense. To see it unfold with such swiftness is surprising, as well.