As a title, Bullyville is borrowed from one of the many nicknames given to Baileywell Preparatory Academy, the all-boys boarding school where most of the novel takes place. As Bart Rangely (a.k.a. Fart Strangely) tells us:
Among the things I never understood about Baileywell was why everything and everyone had a nickname. (1.5)
What Fart, er, Bart fails to mention is that sometimes things have several nicknames, which can be confusing. The school itself is variously referred to as Bullyville, Bullywell, and even Bullyreallywell. Got that? Good. If not, the key takeaway is this: Bullying and Bart's time at Bullyville are key to this book. And that's why Bullyville is the title.