It just takes one

...time losing your temper to set in stone your reputation.
...obnoxious student to ruin your day.
...uplifting letter from a student to remind you of why you chose to teach.
...honest conversation to win over a class.
...teacher who cares to change a school's culture.
...moment to smile at a student entering your class.
...moment of neglect to turn a student off.
...literary work to hook a student on reading.
...year to make a good teacher want to leave the profession.
...timely joke to lighten the mood.
...driven student to salvage a class.
...(fill in the blank)!