Does salutatiion comes before subject in formal letter? accorfing to our school, subject is followed by salutation. Please clarify

Yes, the subject is always followed by salutation i.e. the salutation is written first and then the subject. We kindly request you to follow the link given below to study the proper format of formal and informal letters:
https://www.meritnation.com/ask-answer/question/instead-of-showing-part-by-part-please-show-a-letter-for-sa/letter-writing/3453859