Yes, they do need to learn. Which is why kids are supposed to be reared by adults. It is the adult place to teach a child how to behave. A slap from a peer out of anger is not a form of teaching. All it says to a child is "if I say something people don't like, they might hurt me" not "its wrong to say such a hurtful thing. Not all children are lucky enough to be born into a family that loves them or can take care of them, and that is not a child's fault. I understand you're frustrated with your own home life but it is cruel and unfair to take that out on your classmate." That slap taught her nothing. It will not make her a better person. It will only make her bitter. And that is not how you teach people to become good people.
Maybe it's cultural differences here but, sometimes people just need to be slapped. Just so they can "catch" themselves.
Obviously she's just a little girl, but she was acting like a bitch. What she said was uncalled for and poor Moi already had some family "insecurity" problems as it was.
I deem that slap completely justified. Maybe she'll learn.
Nobody wants to be that person who teaches them. Nobody wants them to get taught. But the world is the world. At some point in their lives, people have to learn that they can't just go around saying shit.