The children about to leave school for summer vacation, providing a joyful tone to the scene.
Who wants to go first?...
Usually jokes are associated with playful and joyous tones, but in this case, the story says "they rather smiled than laughed", which makes you wonder why they aren't laughing, adding mystery to the story, giving off an ominous tone.
"The black box grew shabbier each year: by now it was no longer completely black but splintered badly along one side to show the original wood color, and in some places faded or stained."
The towns people are about to kill this woman for a tradition that goes on every year. It's a terrifying tone as she yells that it wasn't fair as she gets pelted by stones to death.
The box represents some type of supreme decision maker, as it decides who gets drawn to get killed. So it symbolizes a sort of "higher being".
This example of imagery tells that the box has been through a lot, and everyone participating in the tradition respects it to a high degree, as it chooses who lives and who dies.
The whole town really could be a metaphor in a way. The actual differences said throughout the story and how they're the only town that does the tradition compares its values with the rest of the world. The town could also metaphorically represent an outcast society and alienation.