MOM SHE'S BOTHERING ME!

Well the school year has started and with it comes the kids at school that pick on ur kid. Last year on the bus there was this girl who was always picking on ana my 9 yr old and so I would call the mom or tell the bus driver it never really helped but as mom I  did everything but smack that girl in the head. Now for my problem this year there is another girl bothering her I have told the bus driver and she was moved 3 times one the bus and still that girl bothers her. Ana is 9 and she is reaching that age when she has to learn to handle it herself even tho I want to go save the day I cant. Don't get me wrong if something big happens ill step in but for something like this she has to man up and I feel so bad. She keeps coming home telling me about this girl and asking me to tell on her and I cant. so what do u think am I wrong?  The girl isn't hurting her its just words. That look she gives me when I tell her she has to egnore her and figure out how to handle it kills me. This is the worst part of school why do little kids have to be so mean?