Teachers need to go to work believing they can change the world, and then go home and know they can't.
As a teacher of middle school students for the last nine years, I can relate to this. In fact, I experienced a similar epiphany before I even became a teacher.
Let me tell you a quick story:
During my middle school teaching practicum, I was approached by the principal to tutor his son, who was in high school.
Long story short: it didn't go so well. I worked my tail off, but I just wasn't able to interest him in school, nor could I get him through the assignments and tests that he was being graded on.
I would routinely stop and talk to the high school's councilor who was also looking out for the student. The councilor could see that I was greatly bothered by the student's lack of progress, so he stopped me and said, "Look, I can see that you are trying really hard, and that you are frustrated by a lack of progress, but you need to know that I stopped playing Christ along time ago."
He went on to talk about the need to try our damnedest to help every child, but that we can't run ourselves into the ground when we realize we can't save them all.
Rather than seeing this as a way to sell apathy to our consciences, I see this as a way to justify our efforts without demanding perfection.
Also remember that there is rarely ever a timeline for success. At the time, my tutoring and the student's academic career both could have been easily considered failures, since then my teaching has improved and I've heard the student is doing well for himself, too.