“Simply because we were licked a hundred years before we started is no reason for us not to try to win” -- Atticus Finch, To Kill a Mockingbird by Harper Lee
It's part of my philosophy in general that some fights are worth fighting even if you know they're unwinnable. This is true of life, the most important unwinnable fight of all -- after all, nobody gets out of life alive -- and it's true of many impossible things that are nonetheless worth trying to do. (As the admittedly somewhat cheugy proverb goes, "Shoot for the moon: even if you miss, you'll land among the stars.")
I'm still young and idealistic enough to believe that every student is in principle reachable, even if I'm far from expert enough to reach every student myself. The fact is that I don't know for sure which are the students I can reach and which are the students it would take a more expert teacher than I to reach, so I need to try my best to reach every student.
But even if that weren't the case, even if I knew for sure that some students are unreachable by me or anyone else and I knew exactly what students were the unreachable ones, I would still be inclined to try to reach them anyway.
Of course, there is a tradeoff here -- every minute I spend trying to reach an unreachable student is a minute I'm not spending on students who are engaged and ready to learn. I fully recognize that at a certain point, it becomes necessary to cut your losses and concentrate your effort where it will help.