When kids have academic problems, report cards make that clear to parents. And if a kid skins a knee or breaks a bone, parents know what to do.

But detecting that a child is being bullied, and then knowing how to react, may not be so clear-cut.

Kids often are reluctant to tell their parents they're being bullied, making it difficult to know that they're having trouble with other kids at school or online.

One thing that's very clear, however, is that bullying is not a rare occurrence. About one in five kids reports being bullied at school in the past 12 months, and another 16 percent have been harassed online, according to a survey from the U.S. Centers for Disease Control and Prevention. The CDC also found that 6 percent of children and teens didn't go to school at least once in the previous month because they were concerned for their safety.