That said, I were DD's friend's parent, she would pretty much be grounded for awhile.
Sadly, all families don't work that way. I've had plenty of kids (and parents) tell me over the years that schoolwork just isn't a priority for their families. Loads of my seniors put their part-time jobs before school; they'll skip school to work an extra shift, or they'll skip school to work because the boss says he'll fire them if they don't, or they work 'til midnight, then are too tired to do well in school. I've had kids tell me that they couldn't do their homework because they have family visiting and their parents say that they owe it to their visitors to spend time with them. I've had kids stay out the whole week of the county fair because that's a big social event for their families. It's sad, sad, sad.
In some families a D is considered just as good as an A -- passing is passing; why should it matter by how much? In some families it's just
expected that kids are going to fail some classes; for example, I've had parents tell their kids in front of me, "I never understood _____ either -- I expected you to fail it." These are the same families who don't seem to comprehend how grades are averaged; they don't grasp why one grade of 100 doesn't "wipe out" a semester of 50s. They don't seem to understand that attendance is necessary.
These families seem to think that people who get good grades are "lucky" or are born with a gene that they don't have. They fail to comprehend that taking notes in class, reading your assignments at home, and studying before a test = good grades! Instead, they think it's a trick of fate that keeps their grades low while other kids skate by effortlessly with straight As. On occasion, these kids make a good grade on something; they tend to think that was a lucky break or that that particular test was just really easy.
Ponder this too: All kids don't have the goal of graduating from high school -- oh, the great majority do, but a few are there for other reasons: In some cases the court has ordered them to attend school. Not to PASS CLASSES, mind you, just to attend school. If they don't attend, they go to juvenile hall. Others are there because they get free breakfast/lunch and their parents get state money as long as they're students. Same thing about passing classes vs. attending. Some kids are there to sell drugs. Again, these kids don't make up a large number, but they do exist and they take up a great deal of the administrator's time. Typically these kids stay in school until they screw up in some way and they're finally kicked out -- and they seem to expect that their high school careers will end in this way; they're just biding their time until it happens.
A few others -- and the girl you're describing could fall into this category -- are sort of the same: just drifting through, not doing much, not expecting much, just taking the path of least resistance . . . but they aren't troublemakers, just not-too-bright, fall-through-the-cracks types. This tends to be a girl thing. They're hanging around waiting to meet a boy to marry, or waiting to see how long it'll be 'til they get pregnant. Who really cares if you pass Biology or not? Even if you make it past that hurdle, Chemistry's bound to trip you up. If not Chemistry, then English. You could
never, ever manage to pass all those classes for graduation anyway. Other people can do it, but you can't. Just don't bother. You'll never be anything anyway, but you're bound to meet a nice boy in school, and he'll take care of you as well as he can.
By the time I get them as seniors, these concepts are deeply engrained and essentially impossible to change. It makes me want to scream.