It's not wrong to say "good morning girls." But after the students indicated their proper pronouns and asked to please not be misgendered, she should have just said "okay," moved on.

It's not wrong to accidentally misgender someone. It is wrong to refuse to be corrected and insist on continuing to misgender someone.

For example, if someone started call me the name "Dave" I would be uncomfortable. I would probably ask them to stop calling me that. If they insisted on calling me "Dave" or even said they had to "ask my parents" before calling me by my actual name, I would be quite offended. This includes if I was a kid in school. I am not a Dave, I never have been and never will be. I am a woman, and being called by a man's name feels wrong. If a person kept on insisting I was Dave and refused to stop calling me Dave, I would consider that a major act of disrespect. If someone accidentally called me Dave a few times, and if I corrected them and they said "Oh sorry" and went back to calling me my. name, I would be okay with that.

I am a teacher, and there are lots of things to call my students. I often address them as "good morning students!" or "Good morning scholars!" "Good morning mathematicians!" or "Good morning awesome people!" If the kids don't like it, why do it? It sounds like it wasn't just an innocent, accidental misgendering, but more like the teacher was insisting they were girls, imposing an identity onto them.