Why, after fifteen years, does Gene return to school? What does this tell you about how the author will plot the story—that is, arrange its related events?

Gene, the narrator, sets the scene of his return with a long opening descriptive passage about Devon and the town around it. He has come back on his own, because, he states:

There were a couple of places now which I wanted to see. Both were fearful sites, and that was why I wanted to see them.

This tells us that the story will likely be arranged around the events that made these places "fearful" to Gene. We know one site will be the flight of marble stairs in the First Academy building, a set of steps Gene says he climbed every day of his school years at Devon. The second place is on the very edge of the campus, past the main buildings, the secondary buildings, and the muddy playing fields: it is river with a few tree growing by its banks.
We can predict that important action or plot points will take place in these two locales, and, in fact, Gene immediately drifts back in time to a particular tree by the river.