Let's not forget the many students also have R/L issues that may be hard for the teachers to diagnose, and, even if the problem does become suspected or known, may not be possible to fix, either partially, or completely.
One good example of what some of these issues could be, for example, the home and/or other environment(s) surrounding
this. Try giving two plugged nickels about studying with all of
that going on. (I know people that have been through it, and it shows... excellent students with excellent grades, then when the beforementioned is going on, grades plummet, when it (at least temporarily) stops or diminishes, grades rise somewhat again as life becomes a little bit more bearable.)
Other problems could include parents going through divorce / always arguing and creating stressful environment, personal relations with friends / boy/girlfriends going through rough times, etc, etc... there are many many dynamics, and teachers should be aware and try to help, as should parents, as should the students themselves. However, too often, the problem gets mis-diagnosed (student must simply not care, because they are sullen and rebellious) and/or ignored (teacher has too many students to make more than a paltry effort at reaching out to students that are failing). Parents can make the same mistakes as teachers, and of course, some teachers / parents can be part of the problem.
And, let's not forget, yes, there are those people who are just spoiled, rotten apples... the fix for that would be to try and convince/inspire them to make something more out of themselves. Might or might not be possible, but must at least be tried.
It would really help if parents/teachers/principles had access (I'm not sure, do they) to a complete record of any student's quarterly (maybe even monthly) performance in all grades, classes, and which teachers were teaching. Then patterns might emerge (specific teacher? specific subject? specific timeframe across the board? what were the trends before the change?)
[/endramble]