According to the DSM5, the first criteria for a diagnosis of PTSD is that “The person was exposed to: death, threatened death, actual or threatened serious injury, or actual or threatened sexual violence”. It’s a stretch to consider a threat to be sent to the principal as meeting that criteria. That being said, some kids are so sensitive and/or anxious, that their reactions to stressful situations are unusually extreme. It seems that you are by temperament an anxious person. For that reason, it’s possible that the teacher’s behavior was traumatizing to you.
It sounds to me that you continue to be anxious. It could be that you latched onto the incident from so many years ago as a way to make sense of your anxiety. It could also be that you haven’t yet figured out a way to navigate conflict with women and that triggers memories from then.
In my opinion, you’ve suffered with anxiety and the fallout from that incident much too long. You are understandably sick of it. You said that you are often in despair. Regardless of whether an assessment would determine an official diagnosis of PTSD, you are in emotional pain. That is reason enough to get some help.
A mental health professional will be able to determine a diagnosis, of course. But far more important than a label is that counseling will help you learn new skills for managing your emotions and for dealing with stressful interactions with people. You are in your teens. Working these issues through now will help you now and in the future. You deserve to have a life without despair.
I wish you well.