Symbolism, Imagery, Allegory
The school bus is one wicked dragon that the Logan children eventually succeed in slaying.
No, really. The bus is personified as a "huge yellow dragon breathing fire" (1.68), and is the bane of the children's existence, splashing them with mud as they walk the long distance to school. Taylor portrays the bus as a sort of malevolent force with a mean sort of intelligence.
Stacey's plan to sabotage the bus works, and again Taylor personifies the bus: "Then it sputtered a last murmuring protest and died, its left front wheel in our ditch, its right wheel in the gully, like a lopsided billy goat on its knees" (3.88). After the children have taken their revenge, the bus appears more of an impotent farm animal than an all-powerful dragon.
So, why does Taylor build up the school bus to be the Big Bad? Well, we've seen the literal angle: the bus physically hurts the kids, so they physically hurt it. Easy peasy.
And there's a deeper level. (Isn't there always?) By putting the bus out of commission, the Logan kids are engaging in a type of civil disobedience that levels the school transportation playing field between the black and white schoolchildren. For a time, at least, the white kids will get a little taste of what it's like to walk a mile (or ten) in their shoes.
Er...bare feet, in most cases.