They're not speaking as a teacher, but as a person with integrity, someone who endeavors to tell only the truth, especially in a situation as grave as the discipline of a child under their watch.
The very act of doing this seems to irritate the author - which I find very disturbing. Trust, but verify, is the principle here.
Asking the child if what the teacher said is true is seen as an affront on the author's character as a person, not as a teacher, so it's reasonable for them to take offense at the accusation.
It's entirely reasonable not to trust the teacher's authority by default--I'm not challenging that. I'm only defending the author's position on taking offense to being challenged on something that was just said.