This teacher was harming a child. I don't care if it wasn't an issue of hitting the child with a belt, but she was harming this child. The OP tried multiple times to get this teacher to stop. Frankly, I'd be livid if this had been my child, and as it is, I'm sick thinking of what the OP's child (and the other poster's nephew) had to go through. Yelling to protect a child was the right thing to do - made evident by the fact that it wasn't until she yelled that the administration finally decided to do something about it. I don't view this as etiquette as much as an issue of health and safety.
As far as making it personal, it is personal. It is your kid and it is personal. And my husband, a teacher, would agree with me on that - a parent-teacher relationship is very different from other relationships. The teacher is a professional, but for the parent, they are the advocate for their child.
OP, I'm relieved that things are better for your son now. And, honestly, if someone makes reference to what happened before, I think you'd be correct in reminding them that what happened was because you were looking out for your son, and you'd appreciate if the focus could be on what is best for your son right now and in the future, and not on how the school failed him in the past.