Bullying in school

When does having fun cross over to bullying?

I really dread that I now have to face this but I suppose it's something that was probably a matter of time.

For the past week or so Madison has been having these really weird night terrors.

She would go to sleep fine, then at 1130 pm or midnight she would suddenly start playing on the slide in her room by herself! It is really creepy.

When we go up and ask her what she was doing she would start crying inconsolably until I told her to stop it.

The next day she says she doesn't remember crying or doing any of those things.

I asked her if she was feeling sad and she said a little bit. She then told me that there are 3 girls in her class who wanted to use her markers despite her saying 'no'. They then started to ?pinch her on her arm. She told them to stop or she would tell the teacher. They continued to do it so she did tell the teacher and the girls were told off. Subsequently they didn't want to be her friend or something to that effect.

I talked to Madison and said what she did was right, and that she could also give them a warning the next time before going to the teacher.

For some reason the pinching occurred again today! I asked Madison if she wanted me to speak with her teacher and she did.

So I've called the school up and spoken to the counsellor. And I've dropped an email to her form teacher. I'm waiting to see what comes out of it but am wondering how far I should escalate this matter!

I just don't understand why some children can just be so mean and hurtful. Is it a parenting thing? Are these children spoilt? Do they need more attention? When does being playful cross over to bullying? I think if it starts to hurt the other party it can constitute bullying?