Why should he get to graduate when he never came to class? 

Schools have a major incentive to pass their students, even when they don’t deserve to pass and matriculate 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. Long gone are the days of the 19-year-old high school sophomores. If a kid does not complete the work required of him in 10th grade, he’s passed along to 11th grade, just like the rest of his peers who actually did the work. 

When kids face no consequences for not doing their work, there is very little their teachers can do to motivate them to come to class and complete it. There will always be some kids who are actually internally motivated and want to get good grades for the sake of getting into college, but they’re not the ones who make teachers’ lives miserable. The kids who don’t care whether they pass or fail need some external motivation to actually 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. 

The teacher in this story literally isn’t allowed to fail her student, who, in any just society, would fail her class. She’s forced to watch him graduate with his peers because that’s the state of education today, and there’s nothing she can do about it.