It depends what your definition of yelling is and what you are saying. It is different to verbally abusing someone.

You dont know what those kids have done and what has led the teacher to do what she has done and relying on what your child has told you is not always the best thing. Take for instance when you ask them about what they did at school for the day...do you think they tell you every detail on how or why they did something? How are they to recall the exat details and background to a situation thatr isnt even directly involving them?

As Nic said she has only heard it once and the source of her other info is not really a reliable one. \

If she were a good teacher, I am sure that she would have used it as a last resort. I know that I have plenty of other methods that I use to get a point across or to stop a certain behaviour in a positive way before having to resort to yelling at someone.