When Benjamin Zander, the longtime conductor of the Boston Philharmonic Orchestra, found himself a teacher to young students at the New England Conservatory of Music, he decided to tackle the issue of student self-doubt head on. He learned from experience that even the best musicians fear making mistakes and dread the conductor’s evaluation of their performance. So, on the first day of class, Zander would announce to his students that everyone would receive the grade of “A” for the course, as long as they met one condition: the students would have to submit a letter written on that first day — but dated on the last day of the class in May.