A teacher in a dream often stands in for guidance, but also for the part of you that evaluates your own progress, sometimes harshly. Being praised by a dream-teacher can reflect confidence in a skill you're developing, while feeling judged or unprepared may point to self-criticism louder than the situation warrants. The dream usually asks whose standard you're actually being measured against.
The best artist in the world may chance to err, when he mistakes a true significator for a false one. - William Lilly (1602.-1681.)