Teachers are between a rock and a hard place when half their class is multiple grade levels behind and shows no interest in catching up.
It's no secret that many kids are getting left behind in the education system. When I was a kid, America was all about not leaving kids behind in this exact setting (when I was a kid, I thought that policy was about parents not leaving their kids behind at the grocery store), but now that it's been 20 years since that initiative, it seems like it might have been a mistake. I understand why teachers and parents don't want a 16-year-old still in a 5th-grade classroom, even if, educationally, that's where he belongs. But many kids have been promoted when they shouldn't have been, and it has hurt them in the long run.
Schools have a major incentive to pass students, even when they don't deserve to move on to the next grade. They don't want their school's graduation rate to be low. If only 60% of high school seniors graduate, the school looks bad and gets less state funding. Long gone are the days of the 19-year-old high school sophomores. If a kid doesn't complete the work required in 10th grade, he's passed along to 11th grade, just like his peers who did the work.
When kids face no consequences for not doing their work, teachers have very little they can do to motivate them to come to class and complete it. Some kids are genuinely motivated and want good grades to get into college, but they're not the ones who make teachers' lives miserable. Kids who don't care whether they pass or fail need external motivation to get their work done if they're not getting it from their parents. If a kid actually thinks he'll be held back and unable to graduate with his friends, he might become more inclined to do C-grade work instead of F-grade work.
I feel bad for teachers who are thrust into this system without any real way to fix it. They can't hold their students to a high standard because they get in trouble for giving the grades their students deserve. They are forced to hold their students' hands through everything, even if a student will be going to college in a couple of years. When a professor has 300 students in their lecture, they are not going to give every kid unique special treatment.