The girl with the chip on her shoulder flipped me the middle finger today. This was as she played with scissors nonchalantly. She didn't expect me to catch it, but I did. I addressed it privately and she denied it. I always address and then walk away. There's no sense in waiting for understanding when you're not likely to get it.
I then returned to her table twice. The first time, I told her to let it go and move on and the second, I told her it's much easier to be pleasant.
Teacher Tip: Address the situation, but don't always wait for acknowledgement or understanding. That way, students see you acknowledging the infraction, but you're not inviting opportunity for a power struggle.