Agree with all the above.
This is some advice I was given for action a child can take.
It's to be loud. That is, if there is pushing, shoving, holding against a wall, to be loud, shouting Leave Me Alone, Why Are You Doing This, Get Off My Bag and so on.
It's quite nerve wracking but quite often a child wants to shout anyway about what's being done to them, and you sort of give them permission.
The advantages are, the shock for the bully at someone shouting and yelling at them. Drawing attention to the incident. Importantly, if there is anyone in authority around, it alerts them to the fact that it's bullying, not playing, not a mutual fight or argument, but one or more people having it in for another child. I think this is important because it forestalls the teachers being able to shuffle it off as both children's fault. And it's really good for the bullied child's self esteem.
My Y7 child did this and it worked. There was no teacher involvement but the shouting led to a shouting argument and then to talking.