Well, first of all, parents in general need help to recognize the symptoms of depression in their teens. So many parents have a “Oh! Not my perfect precious child!” when it comes to things like Dyslexia, ADD, and Depression, and it renders a grave disservice to the children.
If the child is already prone to dark, depressed, and/or suicidal thoughts and actions, the bullying can be enough to send the kid over the edge.
Other than that, maintaining an open relationship with the child (as hard as that might be) and properly monitoring the online activity of the child (within reason appropriate to the age and maturity level of the child) is necessary. Maintaining a level of communication with some of the kid’s teachers and faculty can help, as well. Teachers notice quite a bit, and have the advantage of relating to that age group on a daily basis. Many become close confidantes of shy or reserved or scared students.