I suppose one could go back and look at popular style guides from the 1980s and 1990s and see if they endorsed it.
And your teacher would presumably have marked down Shakespeare for the same thing. If it was good enough for Chaucer, Shakespeare, and Austin, you'd think it would be good enough for your teacher, but we went through a particularly prescriptive period in the early to mid 20th century (though your teacher was maybe slightly behind the times even in the 80s).