IANAM and IANAL. I am not even sure that the below are the most recent positions. But medical manuals set quite a high bar for the traumatic event that triggers PTSD.
DSM-5:death, threatened death, actual or threatened serious injury, or actual or threatened sexual violence
ICD-10: Exposure to an event or situation (either short- or long-lasting) of an extremely threatening or horrific nature. Such events include, but are not limited to, directly experiencing natural or human-made disasters, combat, serious accidents, torture, sexual violence, terrorism, assault or acute life-threatening illness (e.g., a heart attack); witnessing the threatened or actual injury or death of others in a sudden, unexpected, or violent manner; and learning about the sudden, unexpected or violent death of a loved one.
Of course, as others have said, that does not mean that bullying cannot cause terrible harm, but it seems hard to fit that into the definitions above.