I think the mother is brilliant and 100% correct. She was not endorsing violence, just recognising that her daughter was left with very little option. Could she have run away from the boy? Maybe, but as she'd already told him to stop repeatedly and reported it to her teacher, she was forced to defend herself in that manner. Not ideal, but what else could she do?
I blame the teacher for telling her to "ignore" it (wtf?!) for it reaching the point it did.
Kudos to that strong mumma!