It's so sad that adults are so quick to dismiss the feedback of a small child as them misunderstanding, when I find that all too often that's not the problem at all. Another explanation is that the K teacher is indeed yelling, and either lacks the self-awareness to know that she's doing it, or is self-aware and lies about it to parents.
Another potential source of anxiety for your DS could be coming from the teacher actively suppressing him to get the other students engaged, because he's already reading and they're doing letters still. This happened to my DD in K... her teacher was quite pointed about it. DD came home one day and tried to tell us she'd forgotten how to write an "M," and we were OUTRAGED.
Data is the key to unlocking this mystery. Does your school allow for parental observation?
