While Billy isn't well-schooled or anywhere near a genius, he seems pretty capable with understanding people. For example, knowing that getting Eric to answer a Business Ethics question during the Decathalon would force his rival to flame out.
Billy's former childhood victim, who he made amends with and befriended, appearing at the end to save Billy by shooting Eric. In the ass.
Billy:Man, I'm glad I called that guy.
Billy's classmates demanding the principal to recant his false confession.