I have the attitude of do not be the one to start it, but you may be the one to finish it. If everyone lived by that, no one would be starting it.
If ignoring, walking away, shouting "stop" or telling teachers haven't worked, sometimes retaliating is the only option that helps.
I have slightly built sons. They've been doing karate for years. Fortunately they've never needed to fight back.
I did have to fight back. Once at school. Once as an adult.
At school, a pair of younger girls made the mistake of thinking I was an easy target. Over a few weeks they were not deterred when I blanked them or retorted back, and one day they decided to ramp it up a gear while I was alone at the bus stop. They rather subtly leaned on my school bag hoping to push me down to the ground. I instead released the shoulder strap so the back dropped, the main one fell over, and bonus, the metal buckle swung up and whacked her in the face.
Fortunately the bus turned up in the nick of time before they could respond with anything nastier than "what did you do that for?" which was answered with "you deserved it" and they decided never to go near me again. It didn't stop them making other peoples' lives a misery though.
The second fight back stopped sexual assault escalating into rape when I kicked him in the face. Hard. I hope his neck hurt from the force.
#bekind can fuck right off there.
I have not made a habit of belting or kicking people since but have zero regrets about defending my own safety.
Where diplomacy fails, I don't want my children to be afraid to defend themselves, and so far they've made it to secondary school making sensible, plesant friends and neither being bullies nor bullied. So far whenever they have had issues they have been dealt with by reporting to staff, and that tends to be their first port of call.