DS(6) has been playing up at his karate class for the past two weeks and the instructor told me he is copying other children who misbehave. Last week he and three other children were so disruptive that the instructor said the class was a write-off because the staff put more time into managing their behaviour than actually getting much teaching done. The instructor asked all four of us parents to stay behind at the end of the class and with the children present, read them the riot act and asked us to reinforce this message at home.
I am furious with him and embarrassed at his behaviour. He has been doing so well there (from instructors feedback, parents can’t watch now due to Covid). He knows he must never copy other children’s poor behaviour and yet he’s done it again the second week running.
Any tips please for reinforcing this with him? I’ve asked him if he doesn’t want to go anymore as I am not paying for him to misbehave, nor should other parents have their child’s class interrupted by my child’s behaviour. I’m so annoyed 
FWIW he does four other activities and is well behaved in these so this is not typical behaviour.