DOinit, are you now saying you doubt that the teacher said this? I had the impression that you were pretty sure about it.
Are you saying that if the words reported by the OP turn out to be accurate, even without knowing anything about that teacher or the context or the tone or anything.....that she is a person with a tendency to cruelty, a mean bitch and should never be in the class again? Just how can you reach so extreme a conclusion and judgement based on one phrase (even if it actually was said). I'd imagine you've said some things in your life, as I have too, which if taken in isolation and out of context could show me in a bad light......but I would hope people wouldn't do that, but would look at the bigger picture and make sure they had sufficient information before making such extreme judgements.
Fortunately, if the OP decides she needs to report this matter to the Head (and for all the reasons I've given before, I really don't think there is a need - there is no pattern of such comments and as the OP has made contact with the teacher, I feel pretty confident an explanation will be given and everyone will move forwards, with acknowledgement of insensitivity by the teacher,if there was insensitivity) I can say that the reaction of the Head and anyone else involved WONT be like Doinits. The Head won't decide that the teacher clearly is emotionally cruel as a person, based on one comment, or that she must be a mean bitch, or that she should never be in the classroom again. Fortunately a Head would investigate and gather the facts. If necessary, the Head would probably have a conversation with the teacher about the need to be careful in what she says...and the teacher would probably whole heartedly agree - because there has never been any evidence of the comment being malicious, but an off the cuff comment that was ill-judged, and the thought that she had hurt someone's feelings would probably make her feel sorry. In most cases that teacher would offer and willingly apologise to the girl - there would be no need to force it to happen, because most teachers will be sorry about unintentionally causing upset. And then the Head would remember too that this teacher has always been professional and known as kind (assuming they are the usual teacher - and nothing the OP says suggests a wider, broader pattern of such comments to her child) and simply know through years of experience of dealing with staff and children, that these things happen to even the best of people and teachers....and move on.
In all likelihood, that teacher made an ill judged, off the cuff comment and had no intention at all to cause hurt or embarrassment...and when they realise they did, will feel mortified and happily apologise. If she had realised herself, she probably would have apologised immediately. However, in a busy school day, where 100 things are going on at once and loads of things said to loads of people, it could be quite easy to say something wrong occasionally. It doesn't make it right.....but it doesn't make her a mean bitch, or someone with a tendency to cruelty.....sure
Y everyone can see that those judgements are a massive leap from the very scant information we actually have.