I told DD, "It's not what she may have said; it's what she's written on your homework that's driving me."
In keeping with what others have posted regarding encouraging your daughter to share what is going on at school, not letting teachers divide-and-conquer or come between parents and children, and also possible future impacts of a child feeling stuck to tough-it-out in an intimidating situation... you might wish to consider communicating the above not only to your daughter but also to the teacher and the principal, requesting (requiring) their future communication be solely with you as parents directly, without side comments to your daughter or using your daughter as a go-between.
Clearly setting forth that this is a matter on which the adults will communicate privately, demonstrates that you will not follow in the teacher's footsteps, essentially allowing her to lead and determine the course of the matter. It also avoids involving others (child or adult) who may be within earshot of the teacher's comments. Other children may easily pick up on a teacher's displeasure with a student, resulting in exclusion. Simply put, others will not want to risk being the object of the teacher's displeasure and suffer the consequences.
So many questions come to mind! Once your daughter is out of the situation, has this brought to the surface an opportunity for the school to improve it's educational delivery? Do parents typically have the ability to choose or request a teacher for their child? If so, the school may get an idea of how many parents prefer this teacher's style. Has this teacher enforced the same rules for all students? Are the rules posted proactively? For example, did the teacher send a note to parents discussing that she will be grading not only the demonstrated knowledge of the academic subject but also the adherence to a specific set of rules and conventions as parents have requested that students form these habits of attention to detail? Did parents have the ability to opt out? Is there positive reinforcement for following the rules (therefore could the negative reinforcement of impacting student grades be discontinued)?
