Symbolism, Imagery, Allegory
Wait a second… isn't Tyro a character? Why yes, yes he is. (And yes, yes this means you can read up on him in the "Characters" section.) Tyro is Bart's tormentor at Baileywell, but over time, he comes to represent more than just a regular bully to Bart, who thinks of Tyro as a sort of stand-in for the terrorists who killed his father in the attacks on September 11. Bart thinks:
I would have liked to get the guys who flew into the towers, but they were already dead, so I'd take the nearest substitutes: Tyro Bergen and his friends […] They'd do fine to take revenge on. What the bullies were doing to me was as pointless and heartless and cruel as flying an airplane into a building and killing all those innocent people. (8.18)
Okay, so we're not sure it's accurate to compare one kid being shoved into a locker with tons of innocent people being murdered. But what we are sure of is that for Bart, Tyro's bullying takes on a sort of larger-than-life presence. It joins up with the other attacks Bart's experienced on his life—particularly the terrorist attacks that took his father from him for good—and Bart starts to see Tyro and his torment as representative of just how unfair his life has become. With his dad dead and gone, Tyro stands-in for everything that's gone wrong in Bart's life.