I teach an after-school Uechi program at an elementary. Sometimes parents enroll their children in the classes because the kids are being picked on. Often the children are smaller and quieter than average.
The thing parents believe is that their child will learn defense instantly...although this is definately not the case with smaller and quieter than average kids. They may listen and learn techniques and concepts, however to apply them in a "bully" situation can become more difficult than finding a needle in a haystack.
Parents figure the bully isn't getting what they need at home, that's part of why they're bullies, and there's no use in calling the parents.
If they haven't even tried to contact the parents, or meet with them, then that will be doing the same as encouraging the "bully" to do what they do best...[for those of you who are wondering what that "best" thing is...it is to "bully"]
The victim doesn't want to make things worse by "telling."
Well, how exactly did you find out? My kids have come out and told me...however i am lucky and they feel no threat of the bully due to his [inferior?] intelligence and slow speed.