I am pretty sure your teacher said no such thing. The second sentence in your question is not grammatical or, at the very least, clumsy. What you can say is
Monica is so beautiful a woman that everyone looks at her whenever she walks by.
The construct X is so Y a Z is almost always accompanied by another clause, usually starting with that.
Herman Melville, Moby Dick:
‘Twas not so hard a task. I thought to find one stubborn, at the least; but my one cogged circle fits into all their various wheels, and they revolve.
William Shakespeare, Julius Caesar:
Caesar, now be still. I killed not thee with half so good a will.
Here, the qualifying clause is implied I killed not thee with half so good a will as I now kill myself.
The Scots Magazine, Volume IX:
Since that, the Duke of Parma besieged it in 1587, and found it, even in those days, so strong a place, that in his letters to Philip II. [sic] he complained...
Edmund Burke, The Works of Edmund Burke, Volume 2:
Nothing is so strong a tie of amity between nation and nation as correspondence in laws, customs, manners, and habits of life.