This is why Kai should've just said something like "Try to make more friends in Middle School, I won't have time to see you as much." or something less extreme. I know this is a manga and all, but what she did is just being a bad friend, and would normally just lead to the friendship possibly ending, and perhaps years of bad blood.
Plus, she'll feel really stupid later and it'll be awkward to make up, especially since making friends with an entire class is not only improbable, but something you probably don't want to do due to there (almost) always being some bad or seriously bad eggs in any given class. Creepy people, stalkers, users and abusers, future serial killers, etc.