Grammar Nerd, I've already cautioned her. As for escalation, DD said that on the fated bus ride, Kayla turned around and told Nice Kid Sister not to play with DD anymore, because Kayla and DD weren't friends. Nice Kid Sister said, "I don't know what you're talking about." and kept talking to DD.
DD believes Kayla was already "upset" with her because at lunch, earlier in the day, one of the more popular girls in class had complimented DD on her new t-shirt. It says "peace" in about a dozen languages in glittery script. Kayla said, "So, what, it just says 'I love you' over and over." DD said, "No, it says, 'peace.'" Kayla told DD off for "trying to be right all of the time." DD shrugged and went back to eating her lunch.
I did give DD a high five! I am so proud of her as a few months ago, Kayla getting mad at her or threatening her with "we're not friends anymore" would have sent DD into a tizzy. Now that DD has more friends and is a little more confident about people liking her, she's able to shrug it off.