I ignored them constantly for six years, didn't work. But do some preplaning,
first step is allways to contact the teacher and let them talk to the class,
if they can't do anything, include parents, both yours and the bullies.
Warn you parents that you will use force if it's not working.
If that isn't working, silently provoke them where you have plenty of witnesses. Don't walk away, don't aswer, and don't show any reaktion whatever they say, just keep looking at the leader. (Make sure you're standing with you back at or close something) If he pushes you (into whatever you're standing in front of), still don't do anything.
But the second he grabs you (make sure you're in range), trash him. Physicly. With all the might you have. Make sure he and his minions knows that you are the toughest and the only reason they are still have their health intact is becouse you spare them.
That's why I held out as long as I did, I didn't want to hurt them. (Ironi for you. )
And for the aim, if you punch him, aim for the chin. If you aim right even a weak hit will make him dizzy (very good for follow ups). A good one knocks him out. If you do knock him out, don't do anything more. That would get you in trouble.
The key is allways to take the leader. If he(or she) stops, the others fall in line as they allwas do.
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