Not all professionals behave professionally all the time. Why is that do hard to believe and decide to conclude the OPs child is lying.
I know if I was misbehaving and was called an idiot, my DM would say it was my fault, but I wouldn't say that to my DC.
It may have been acceptable decades ago, like corporal punishment was, but not in this day and age.
I had a mealtime supervisor say something quite rude to my DD and she wasn't even misbehaving.
It was raining during break time and DD and her friends wanted to go inside. She then says "maybe the rain will knock some sense into you".
I was far from impressed and called the school the next day. The HT said if that was said, it was wrong and got the member of staff to call me.
She tried to deny it, but I told her my DD had no reason to make it up and I believed her. She then proceeds to apologise if there was a misunderstanding.
Never once in 6 years of primary school was I called in or told about DD misbehaving.
I know teachers want to defend their fellow teachers, but as someone who works in HR and has sat on many disciplinary hearings, their behavior does fall below the professional standards at times.
Just as doctors, police officers and other professionals behaviour constitutes misconduct at times.