When writing a letter to anyone, you always include your address. This is so that people can write back to you, or (if necessary) return the letter to you if it has not been correctly delivered.
In a formal business letter to someone in an organization, you usually also include the name, job title, and business address of the person you are writing to. This is to ensure your letter lands on the right person’s desk.
If you are writing to your Principal as Principal, you should treat this as a formal business letter.