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Elizabeth Landau, CNN -- Bullying involves a repeated, harmful, aggressive act in a situation where there is an imbalance of power, said Cheryl Rode, director of Clinical Operations at the San Diego Center for Children in California.

Parents should be especially alert if their children come home from school with torn clothing, bruises, cuts and scratches, experts said. Loss of interest in schoolwork or a refusal to go to school are warning signs. Ask your child what goes on at lunchtime, Rode said.

If you see these kinds of behaviors and aren't sure what they mean, talk with the child's teacher about what he or she is observing about your son or daughter socially and emotionally, she said. Also, speak with the school counselor.

It's time to get a mental health professional involved if these physical or mental symptoms don't go away for more than a couple of weeks, and if they continue to interfere with a child's ability to go to school, play with friends and engage in other activities, Rode said.