After a trauma, over time, a child's functioning at school or home deteriorates as attention wanes and distress becomes paramount. Symptoms of post-traumatic stress disorder and related disorders appear, often months later, and can include withdrawn or aggressive behaviors, difficulties with sleep and eating, stomach pains and headaches, tearfulness or severe anxiety.
Most kids don't tell their parents about bullying. Maybe its due to feeling embarrassed or defenseless, or feeling that nothing can be done, or feeling that they will get in trouble for telling their story. For these reasons and more, parents need to maintain open lines of communication with their child.