One thing about working with kids, they're always telling on each other. This holds true on the school bus as well, however, since you also have the distraction of THE ROAD to deal with, it can be difficult to tell who is lying and who is telling the truth.
When one student starts to tell on another, I usually just ignore it or pretend that I can't hear them and they always move on fairly quickly.
The other afternoon, I hear "bus driver, Jacob is spitting"
Not wanting to clean up spit, I say "Jacob quit spitting"
Jacob says "I'm not spitting"
Not quite sure who to believe, I say "Jacob, I can tell when you're lying"
Quick as a rabbit Jacob shoots back "well I'm putting it back in my mouth"
All I could do was shake my head.