Short Forms...........

Abbreviations such as Mr, Mrs, i.e., and etc. are so common and widely understood that one almost never uses the full words. If one were to write one of the above words in full, a native English reader may understand you, but they'd think it strange that you'd used the full word, and they might assume that you're not an expert English user. Unless you want people to make that assumption about your writing, I would heed your teacher's advice.

It's normal to write the full word 'mister' when you're using the word on its own, i.e. not as a title, but there's rarely a reason to do so. You would also write the full word when writing song lyrics beneath musical notation; you would do this so that you could assign each syllable to a different note.

You would rarely use Mrs in its true unabbreviated form (mistress) because mistress can mean something quite different to Mrs. If you wanted to write Mrs in expanded form, you would more normally write 'missus'. However, as with mister, there are very few circumstances in which you'd want to use the word on its own.

Some words and idioms are so frequently abbreviated that the full form may not be known to many readers; 'i.e.' is such an idiom. If you were to write 'id est', many readers would wonder what you were talking about.

I would not group St with Mr, Mrs, i.e. and etc. When using saint as a title, 'St' is indeed more common than 'Saint', but your average reader wouldn't think it wrong if you used the full word. Failing to use the abbreviation might only be considered strange if you were using the title frequently. In such a case, a reader might only consider your writing to be very formal or zealously religious; they wouldn't necessarily think you were a non-native speaker.

Thanking you a lot Mr.Futurist.

Glad to be of help.