From Paul's description in the story, it sounds as if he may have had Attention Defecit Disorder.

According to Children and Adults with Attention Deficit Disorder (C.H.A.D.D.) homepage, some of the symptoms of ADD include "(1) fidgeting with hands or feet, (2) appearing not to be listening to what is being said, (3) and doing things that are dangerous without thinking about the consequences." Paul was described as doing all three of these things within the first two pages of the story.

1. "Older boys than Paul had broken down and shed tears under that ordeal, but his set smile did not desert him, and his only sign of discomfort was the nervous trembling of the fingers that toyed with the buttons of his overcoat, and an occasional jerking of the other hand which held his hat"

2. "In one class he habitually sat with his hand shading his eyes; in another he always looked out of the window during the recitation..."

3. "The sound of an approaching train woke him and he started to his feet, remembering only his resolution, and afraid lest he should be too late. He stood watching the approaching locomotive... When the right moment came, he jumped. As he fell the folly of his haste occured to him with merciless clearness, the vastness of what he had left undone." (Paul's Case Willa Cather)

The C.H.A.D.D. page also says that

"ADD students have a greater likelihood of school drop-out... academic underachievement and social and emotional adjustment difficulties. This is probably due to the fact that ADD makes children more susceptible to failure in the two most important arenas for developmental mastery -- school and peer relations.

"Making and keeping friends is a difficult task for children with ADD. A variety of behavioral excesses and deficits common to these children get in the way of friendships. They may talk too much, dominate activities, intrude in others' games, or quit a game before it is done. They may be unable to pay attention to what another child is saying, not respond when someone else tries to initiate and activity, or exhibit innapropriate behavior."

Another C.H.A.D.D. page said that when ADD children become ADD adolescents they are usually less visibly hyperactive, but they tend to be fidgety, restless, often looking about, and loud. Because of this their learning skills do not properly mature, causing more academic problems and highly under-developed social skills, which results in even worse peer relationships.

It is because Paul fit into so many of these categories, and the fact that Willa Cather was a teacher herself (where she probably saw quite a number of ADD or ADHD children even before the disorder was discovered), that I believe that Paul had Attention Deficit with Hyperactivity Disorder.