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[berolina]

I was told in school, in writing it should always be it is where in vocabular one can say it's! Unless my English teacher was pants!

I think I know what your teacher meant.
My teacher always wanted it is instead of it's and do not instead of don't and things like that in written formal English texts, essays etc. Of course he was Canadian, but I guess that doesn't matter.
So either your teacher was part pants or your memory's fading