That teacher could lose their job if something happens and they didn't do anything to see that this doesn't get any worse, they may be right, but what if they are wrong. How will they feel if they find out that this kid did something foolish, someone winds up really sick or worse, and they have to live with the knowledge that they knew about the possibility but just blew it off!
If they don't listen or do anything about it, move on to the next adult..someone is bound to get their head out of their britches eventually and do something!