I would enroll my kid in a practical fighting course and motivate him to master it.
Then he would dominate the bully(s).
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I taught my son to kick in the right place. End of that bully and future bullies.
Too many people on this thread are living in the past. The schools are filled with ferrels now and the advice being given will get kids knifed, shot or beaten to a pulp by a gang. The days of hurt them so they won’t hurt you are gone.