Even if the child is in therapy due to the bullying, the child's crushed self esteem can overwhelm the child to the point where therapy may be too late. It will take quite a bit to undue the emotional collapse of the child's self worth. The goal is to catch the bullying early and notify the school so safety measures can be placed to ensure the child's emotional well being is managed.
If therapy is an option it is important to place the child with a therapist who deals with trauma. Trauma therapists have a better understanding of how to deal with traumatic events and focus on those events in an effort to relieve the stress so the child may start to feel strong both physically and mentally.