The narrator says that school is out, but as for the children, "the feeling of liberty sat uneasily on most of them." Usually, when summer vacation has arrived, children are thrilled at the thought of this liberty, but not in this case.
The boys begin gathering stones and this would be ominous if, by this point, the reader knew what the stones were for. But up to this point, the reader only knows that there is to be a lottery. It is a nice day, school is out for summer, and boys are gathering stones. The word "uneasily" is subtle, but it suggests that there is something holding them back from fully embracing summer vacation. The children are still talking about teachers and books from the past year. Why aren't they talking about what they will do over the summer? This, again, is subtle, but significant.
The men make small talk and quiet jokes to which they "smiled rather than laughed." Even the men are uneasy about something. Again, the pile of stones is mentioned, but still no mention of their purpose. The women also engage in small talk. Sometimes, people engage in small talk because of a nervous tension in the air.
Slowly, everyone begins to gather together. This all seems like some type of festive occasion, but it is oddly subdued. This is the subtlety of Jackson's foreshadowing. Withholding the purpose of the stones heightens the tension and foreshadows an unknown "something" to come. Heightening the mystery and uncertainty of what's to come is part of this foreshadowing.