This is a great question. The setting of the story does not lend itself to a ritualistic murder. There are at least three points in the setting that fool the readers.
First, the settling is idyllic and serene. The setting suggests a pleasant story. There is nothing ominous. In fact, the story gives the impression of a perfect summer day. It begins with these words:
The morning of June 27th was clear and sunny, with the fresh warmth of a full-summer day; the flowers were blossoming profusely and the grass was richly green.
Second, the children get out of school for vacation. They are excited about summer. The reader gets the sense that they are ready to play. Again there is no sense that the children are actually excited to stone someone to death.
Finally, as the men and women gather, they do not give the impression that they will be a murder. The men talk of farming and tractors; the women catch up on gossip and other news.
In view of these aspects of the setting, the reader does not expect the ending of the story.