As a teacher, I have been in this situation far too many times. We, as teachers, find the education of students very important (given our choice of career). This said, we have a hard time when others fail to value education, especially parents.
Teachers must always remember that there is only so much they can do. This said, teachers do have complete and utter control of what students experience within the walls of a classroom. If parents are not supportive, teachers must do everything they can to insure that students recognize the value of an education.
Remember, teachers are in students lives for a minimum of 12 years, 9 months a year, 8 hours a day, 5 days a week. Students are in the care of teachers for a majority of their developmental lives. If teachers can impact the students, a parent's lack of involvement will not matter (it will be the student who takes responsibility for his or her education). Although it is always wonderful to have parents involved in the learning process, this is not always the case, and teachers can only try their best.
A link to a document which speaks on improving parental engagement is provided below.
I have found that in teaching, this situation arises more that I would like. However, I have also found that one caring adult is usually what a student needs to be successful. Yes, they will struggle, but knowing that someone cares is a huge boost. I usually took these students aside quietly and made clear that I believed in them and their abilities, and having parents who didn't care did not have to ruin their futures. In parent conferences, IEP meetings, informal team meetings, I voiced my opinion and support for this particular student. Another team member of mine, the math teacher, had an affinity for a slightly different sort of student, recognizing that he would have been labeled EBD himself when a child if he had been born later than he had. Together, we often would keep each other working with a particularly difficult student because one or the other of us truly enjoyed and liked teaching this student. That helped the other teacher to hang on until we could see what the other teacher saw in that student. The people I am called in to see in the jail often tell me that they quit school because not one person cared. I believe that one person can make a big difference in the future of one student.