Gee…you go around teaching kids that the goyim are “subhuman” and it’s ok to cheat and steal from them…then you wonder about behavior like this. Because the kids think that they can act like animals and their being “frum” automatically makes them better than the rest of the universe, without the refined behavior to back it up.
Kids are remarkably observant and will pick up on ideas and modes of behavior no matter how subtle. So the child who hears his father talk about the “shvartze” in his office and how dumb he is will learn it’s ok to talk/think like that. Same with how people from different frum sects are treated. When the childs school is pushing their own brand of “Torah True Judaisim” with no respect for anyone else, its no wonder why they would tease other kids about things like that.
But yeah, it’s much easier to blame entertainers who are perceived to not be frum.