Are you hard of reading? He did show it to the first teacher and that teacher told him to not show it to anyone else.
Then the first teacher should have confiscated it and told him to get it after the school day. High schools these days are much more wary of items than when I went to school. Ask any Middle Schooler around us if they would EVER bring such a device (unless it was an APPROVED science experiment) and the answer would be no.