I have no problem with yelling.
Uncontrolable screeching on a regular basis, I would have concerns.
The kids are ol dnough to know how to behave, and old enough to deal with punnishment. I was yelled at at school, in a class environment, many many times, and there was always a reason for it. It got our attention and I still remember it, so it has a long term impact.
There is huge difference in my mind between abuse and yelling. Obviously had I been the victim of such abuse, maybe there wouldnt be such a huge difference, I dont know?

I think give her the benefit of the doubt. If you are concerned maybe mention that you overheard the exchange and hope that the yelling isnt a regular part of the class. Maybe that will remind her to be more aware of her levels.

As for the rumors of how she feels teaching the class, dont buy in. and dont allow you views of her to be coloured by idle gossip. The woman is a teacher and human being and deserves to be respected by the parents as well as her students.