For parents willing to face the facts and tackle the issue, experts offer these tips:
"You just can't go up and ask (your child), because they may not know," Borba said.
A variety of factors can drive bullying, from peer pressure to mental disorders. And correcting the problem will take some time, Borba said.
Have a talk: The government website stopbullying.gov suggests asking your child for her account of the situation. Calmly explain what she is accused of and ask for an explanation of the incident. Be objective and listen carefully.
Set realistic consequences for your child as soon as you learn of the bullying, said Kristin Moorehead, a marriage and family therapist and founder of Sage Therapy Center in San Diego.
"It's important for parents not to be in denial, but also not to overreact," she said.
Consider turning the situation into a lesson by having your child read bullying-related research and news so she can understand the effects of her behavior.
Take away the tools: One of the most common forms of bullying is cyberbullying. If this is the issue, take away the tools, said Mary Kay Hoal, founder of the teen networking site Yoursphere.com. Then educate your child on the social and legal ramifications of online bullying.
Assess your parenting style: Kids' social behaviors are often influenced by their family dynamic, said Anita Schimizzi, a psychologist based in North Carolina.
"When bullied at home, you often go and bully others," Schimizzi said.
Militant parenting styles can teach kids that combativeness is how to handle conflict. Alternatively, parents who are too lenient can give children a sense of entitlement.
"A lot of times it's hard to have the skill to look at your parenting style critically," Schimizzi said.
Parents need to be honest with themselves about how their parenting may be contributing to the problem. If you're not sure, consider asking for feedback from someone whose opinion you trust.
Enlist the help of professionals: Meeting on your own with parents of the kids who have been the victims of your child's bullying could worsen the situation, Borba said. If you can, invite them to sit with you and school mediators, who can help everyone find solutions. A mediator also can help your child understand the pain that bullying causes.
It may also be necessary to visit a professional therapist or counselor, Moorehead added.
Just don't hesitate to act, and act fast. "'If only' are two words I hear most from parents," Borba said. "And it's deadly."
Here are some online resources:
Michele Borba's blog: micheleborba.com/blog (type "bully" in search field).
Stopbullying.gov, a government site with plenty of tips.
Mary Kay Hoal's teen networking site includes a cybersafety blog: internet-safety.yoursphere.com
Other websites worth a visit: bullyingstatistics.org and http://www.safeyouth.gov.