Heather, I know when we feel our children have been mistreated (raised voices, punished etc.) by some one other than ourselves the "mother bear" comes out in us....
At this point you really don't know what went on in the class room....but it does require a trip to the teachers room to clarify what did and it will also put her on notice that you
will be watching....if she said "don't tell..." that is clearly out of line....
As for the "why's" as to her yelling, there are a lot of variables that could have went on, some kids are just more sensitive than others, so "yelling" to them is just us speaking in a stern voice, the kid gets his feelings hurt in the process and he calls it yelling....was he ignoring her directions?...Maybe maybe not....
We all as moms have had our share of good teachers and some bad ones, so until you get some answers try and remain calm til you do some digging...