You can first educate your child that this kind of behaviour is disrespectful towards the teacher. And from my own experience, I always make my kid understand 'right and wrong' giving her examples where she and I are the characters. So, you can try doing the same, as how would he feel if someone does that to your mom or dad!
Rest, you can meet the teacher or the principal and discuss the concern and can have the agreement that they will just hold some sessions where they would have discussion over moral science. Where kids can be told that teachers are aware of everything they do, not citing the particular example. So, the message can be put across the kids without your kid or you being in the scene anywhere.
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