There are perhaps some cultural differences here, Kenlee. To the British way of thinking not acknowledging a friend's parent would seem extremely rude, to do it is just normal, doesn't mean you are unusually well mannered. And certainly doesn't mean you are otherwise well behaved.
My ds has had a number of detentions for not doing homework and playing around in class and I fully agree with the school in dishing them out. I'd still expect him to have the basic manners of saying hello to people.
But that is not the point here.
The point is that you do not know the back story.
There may be all sorts of reasons why this is a sensible punishment for this particular child. She may have behaved very badly, or it may simply be a case of her needing to stay in school to catch up with work she is not doing at the times she should be doing it.
That is the rationale behind ds' detentions: it's not that the teachers think he is a nasty or rude boy, but that the work needs to get done.
It may be that this has been discussed with the girl and she has already had several warnings.
You simply don't know.
Also, don't forget that because your dd is a long way off you probably feel more protective of her and more inclined to cherish every moment with her. Parents who have their dc closer at hand can afford to be a little tougher or a little more casual because they get many more opportunities to demonstrate their affection. So they may come across as less caring even if they are not.
My db was horrified when I told him that my dc occasionally got on my nerves: he declared solemnly that he could never feel that about his son. But then he lived in a different country from his son and only saw him every few months so every moment they had together had to be a precious one. I lived in the same house as my dc and saw them every day, so we had plenty of times for all kinds of different moments.