More vs most

As a new (addicted) listener I've been feasting on the archives. This one dates back to 2010 in which a listener objected to the use of more to describe seemingly more than two things. "This student is one of the more difficult students I've taught." I see it this way. The student is a member of the larger set of difficult students. the speaker is comparing the two sets of students; difficult students and not difficult students. In this way, more is correct. The student is a member of the group " more difficult students" vs the other group "less difficult students