A good mantra once shared to me by a mentor was: "Teach the kids who show up." Alternately this person would say "Do what you can do for the students in front of you and ignore all else."
I always hated the advice to "just do what you can" because that is not the goal of education. I would say "But we have a curriculum! Players need a solid foundation! We need to master one skill before moving on to the next, and blah, blah, blah."
That was true, but idealistic. Education as a concept is indeed idealistic. Schools are not. They are hotbeds of chaos and confusion, and we all need a device for blocking out the noise in order to teach. Mine is "teach the kids who show up." It focuses my work and facilitates a pleasant, productive class period, one group of students at a time, in spite of the disorder on the other side of the doors.