Well, when talking about the general behaviour of any teacher, either singular or plural works and the meaning is close enough, I guess. But note that Turkish too can use the plural: Öğretmenler kahve içer, so there's no reason not to do a translation that matches the original sentence 1-1.
So, for this one, the options are either "the teacher drinks coffee" or "a teacher drinks coffee".
If the plural does not show on the subject, shouldn't it show on the verb?
http://www.turkishlanguage.co.uk/aoristpos.htm I think the preference is to use the plural ending on the subject. but not bother with the plural ending on the verb. My memory is of " Filler su içer." when I was expecting "Filler su içerler."
I think the general plural is shown with the singular noun when used as a direct object. https://www.duolingo.com/comment/7736911