Generally speaking, while you do need general education, intelligence, knowledge of current events, etc., for general transcription, there is no special school or training required for general transcription.
Legal transcription almost always requires, in addition to all of the general transcription skills, previous legal (usually law office) experience. You may find courses called “legal transcription” courses. These may or may not help you in obtaining legal transcription work.
School/training is required for medical transcription.