I'm a librarian who loves anime, manga, and reading a wide variety of genres.
My guess is that Neil is secretly dying. Many mentions have been made of his diabetes and his limp (which requires surgery that he says he and his mother have almost saved up enough for him to get). He is also losing hair and weight fast enough for it to be noticeable. And the task the Caretaker has given him? Get sick in class.
The question is whether he's also the Caretaker. Possibly doing all of this in a final effort to punish those involved in the desert dude's death?