Help your child firstly to stay away from such problem creating kids. Send a note to teacher asking them to change your son's seat away from where he is.
Sooner or later, these kids will get in trouble and you wouldn't want your kid to be in the middle of this. Such kids can get others in trouble just to get out of their mess.
Regards to telling teachers or parents...u need to guage the situation that it doesn't get ur kid in trouble.
U can write an anonymous letter to the principal revealing tht this issue exists. The managment will do the needful.
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